All Wheel Drive Auto: Independent Seattle Subaru Service


Subaru Check Engine Light and Code # P0420 Explained

If the check engine light comes on in your Subaru there is always a corresponding code set in the computers memory.

You can read about how the system works here.

Subaru Check Engine Light Part 1

Subaru Check Engine Light Part 2

One of the more common codes we see on the Subaru, is P0420 Catalytic Converter Efficiency Below Threshold.  I am taking a more technical approach than normal to describing what happens when the code is set, as it is a very technical process that needs to occur to get real results, we don’t always replace converters, sometimes its just a small exhaust leak, or a failed sensor of in some cases a faulty fuel pressure regulator.  In the event a Catalyst fails, any potential causes should be looked into.

This will hold true in most all of these Subaru models:

  • Subaru Outbacks
  • Impreza
  • Forester
  • The Legacy
  • And WRX model

Its the single most ignored code, and one the the most challenging to accurately diagnose.  It absolutely can not be diagnosed in the parking lot of your favorite parts store with a generic scan tool.

Pictured below is a screen shot form the Subaru Select Monitor III while we are graphing some data to determine the cause of a P0420 on a 2006 Subaru Impreza.  I have outlined the rear 02 sensor in yellow, the pattern we are seeing indicates either the Catalyst is not reaching the point of “light off” or “function” or the rear 02 sensor is sending incorrect data to the ECM (Engine Control Module) .  What must be done next is to determine the correct answer to that question, by testing the rear o2 sensor and by performing a catalytic converter efficiency test and evaluating fuel trim.

Subaru SSM3 Screen Shot, Rear O2

Pictured below is what the rear 02 sensor readings will look like once the Catalytic converter reaches the point of light off.  You can clearly see the differences in the two high lighted readings, I have also made some changes in the time and division portion of the Graph to help better show the data in question.

Subaru Select Monitor 3 Screenshots Good Rear O2

Why the data above is important;

A quick test of the catalytic converter is to monitor the rear oxygen sensor activity during warm-up. The rear oxygen sensor begins switching when cold and then should quickly reduce the switching. The point where the rear oxygen sensor becomes a straight line is called the catalytic converter “light off” point.  It is important to verify catalytic converter light off prior to determining if a catalytic converter has failed. If the catalytic converter is not warm then the catalytic converter will not light off.  The converter would not light off if it was too cold or if the air fuel mixture was too rich or if it is defective. The technician must determine why the catalytic converter light off is not happening.

So at this point all that has been learned is that the ECM is correct and there is a function issue, but until more testing is done we don’t know the why.

Here are some terms;

  • HC= Hydoro Carbon
  • CO=Carbon monoxide
  • NOx = Oxides of  Mono-Nitrogen
  • H20= Water
  • CO2= Carbon dioxide

I would like to add the following

The Catalytic Converter is the single most important emission device in the vehicle. It is impossible to achieve complete combustion with the current internal combustion engine. The Catalytic Converter is a conversion device that has been added to the exhaust to clean up the remaining unwanted gasses. The purpose of the catalytic converter is to reduce HC, CO and NOx. In order for the conversion to work efficiently it needs lots of oxygen. The converter is made of a honeycomb ceramic structure held in place with a Nickel support shell, all contained in a metal housing. Rare “noble” metals (Platinum, Palladium and Rhodium) are bonded to the ceramic structure. These metals give the catalytic converter a 3- way operating characteristic that can change CO, HC and N0x to inert gases like CO2.

At about 600 degrees Fahrenheit the Catalyst hits the light of point, and the following should occur .

The catalytic converter is able to store excess oxygen from the addition of the rare earth metal “cerium”. The Cerium acts like a sponge and soaks up the oxygen when the system is lean and releases the oxygen when the mixture is rich. This makes the converter more efficient at “oxidizing” HC into H2O and CO2. If cerium is contaminated or the converter overheats, the converter is damaged. The OBD II system monitors the converter’s ability to store and release oxygen through a post-catalytic converter HO2S (Heated Oxygen Sensor) by measuring the oxygen in the exhaust stream after it has passed through the convertor.

If the Catalyst is restricted, in the way of carbon deposits, it wont reach temperature and as such wont function, it is possible in some cases to de-carbon the exhaust system, but only if that is the diagnoses.

The next part is a little less scientific and a little more in the grey area, its entirely possible for a check engine light to occur after the battery is disconnected and set a PO420 even though there were no indications of a catalyst issue prior.  The why is learned data that has been lost typically with light of parameters, coolant temperature and maybe engine repairs that make changes in Combustion temperature   such as replacing leaking head gaskets.  But I will save that for another article.

The point I am trying to drive home is its not simple, its not possible to diagnose without testing, its instead at that point called “guessing”.  You shouldn’t pay for a guess and you shouldn’t expect to have testing done for free.  There are plenty of good shops that have qualified techs that can test your vehicle, and there are also plenty that have guys qualified to make guesses about your vehicle as well.

Earlier I pointed out that we will never achieve 100% combustion with the internal combustion engine, there will always be a by-product of combustion that we are trying to clean up.  Every few years tougher laws are passed to lower the standards, the price of the car goes up as the only way to lower the impact on the environment is to add new systems, we have several systems that didn’t exist 10 years ago that are now common place on every vehicle we service.

Thanks For Reading!



199 Responses

  1. Hello! I have a 2017 Subaru Crosstrek that threw a P0420. Got the cat replaced under warranty. Light came back on a month later… same code. Replaced the downstream O2 sensor since it also showed a code for that. Light is back on. Do I need to get the cat replaced again or the other O2 sensor?

    Thank you

  2. Hello I keep coming back to you so now I’m going to ask.
    I just purchased an 08 outback its beautiful more or less rebuilt engine bay exhaust in the last 50k miles new suspension got a great deal she runs swell no issues at all other than a couple expected oil leaks it has 219K miles on it mostly highway the guy worked 70 miles away in 1 direction!!!! Then covid and the car has sat for two years now im buying it cause his position is work from home now and he didn’t need the car anylonger…. So ran beautifully for a week after driving it 4hrs from Atl back to southern Al. I work from home so I don’t drive a lot myself. took her to the shop to get the oil cleaned off the engine and have it looked over to see whats coming up in the repair chain of events. Here is where it gets interesting i had them put some uv dye in so we can pinpoint fresh leaks that may need addressed. Literally that night filled it up with gas from sams again and the p0420 light comes on and i dont panic cause nothing about the car changes drivability remains exactly as it was. i go have the code cleared maybe it was just burning some stuff off. 3 cranks later it comes back on… i went and had it tested this past Friday with a huge obd2 scanner that shows alll the runtime data the and while we were looking at read outs the code auto cleared like completely Vanished and all read outs looked correct especially now that i see your diagrams the o2 sensor read out nice and stable like when the code cleared itself im presuming this is the off position ? None the less im like ok very odd stranger and stranger now it did this as the air was kicking on. Now i do all this reading and searching and blah blah. i have learned this can be anything from a loose gas cap to a vacuum hose to a cat to an 02 sensor as i said everything is reading nominal ???? what does one do i am going to buy a digital full read out obd2 connector for my phone i can leave attached to get some live data recorded but im currently left scratching my head as to what to do now … Other than start collecting live data snapshots of the moment it shows up. to hopefully pin it down

    1. Hi Jacob,

      I’m a little bit at a loss on how best to help you here. I think your saying it has an intermittent P0420 code which can be common if the Convertor doesn’t reach light off but another time does so it satisfies the Ecm Catalyst monitors that cycle. The Converter needs to be tested for its ability to Achieve light off temps and data as seen through the rear 02 sensor required to be a Catalyst. Thats the only next step. If it does than its possible something secondary is at fault. if it doesnt well you need a Cat convertor.

      Hope that helps


  3. Hi Justin/
    I found you doing research about my P0420 code on my 2015 Subaru Forester turbo xt. I did Buy it used about 16 months ago at only 80,000 Kms. She was perfect. Love the turbo. Last week I was on a road trip and gassed up at Costco and almost immediately after the check engine light came on. My 2015 only has 110,000 Kms (80,000 miles) and I have only ever put in premium gas and always do premium oil changes every 5000 Kms. Up until now my Subaru has been a dream. Even with the check engine it was running perfect until it didn’t. The gas dropped like crazy and it started smoking exhaust. After the check engine came on we took it to a friend in the small town we were close to – mostly because when I called thelocal Subaru they immediately told me over the phone I had to get the entire turbo engine replaced. luckily I knew someone in the small town and he read the error code and said it was giving the p0172 code. He reset and said it was fine to drive back home to my regular mechanic. My long time mechanic said it was now the p0402 code and replaced the o2 sensor. I got everything fixed and filled up with the best premium. everything worked fine for the first 300 Kms. Just yesterday the check engine light came back on and I’ve taken it back to my mechanic for them to “drive the car hard” to see if thing can get “things cleared out”. They are now taking about replacing the catalytic converter. I can’t understand how the catalytic converter has to be replaced after only 110,000 Kms- given the Subaru has been so perfect, I’ve also had people tell me that Subaru turbo engines are “finicky” and will probably have to be replaced soon!? Can you give some advice?? I feel so many people – so many opinions,,,,. And so far $1400 in repairs and the check engine light is still coming on…. Thanks for your insight and expertise.

    1. Hy Cynthia,

      Glad you enjoy the XT, they are really fun cars.

      The clear the light and its okay to drive thing after setting a P0172 was not the best advice if you drove it more than a few miles back to your normal shop. There was some sort of a condition that actually needed to be diagnosed and driving it like that did not do the Catalyst any favors.

      Anyways what to do now?

      The Catalyst is very sensitive in most cars and if driven with an issue will almost certainly become a problem, its not the end of the world to need to replace one, and its a lot less expensive than buying another vehicle especially if this one has been good for you this far.

      Yes the Turbo equipped Subarus can and will have more engine related issues than a NA (Naturally Aspirated) engine will have but that’s true for every car and its also mostly about maintenance habits.

      80k is still very low mileage for a Subaru and its got a lot of life left in it. If the Forester still suits your needs over the long run, it will cost less to maintain and make repairs as they come up as the car continues to age.

      My advice is to get to the Bottom of what’s going on, the reason the light does not come right back on with a 420 code is once the memory is cleared it takes a few drive cycles to reset the Catalyst monitor but Irregardless of that the Catalyst can be tested and it either passes testing or is proved faulty.

      I hope this helps and they get to the bottom of it for you.


  4. Still getting P0420 after replacing both cats, front cat and rear cat on 2008 subaru legacy. I cant tell why its giving this code. Two California emissions cats and two brand new Denso O2 sensors. A few weeks later, P0420 came back. What can I check next?

    The front cat had some of the honeycomb broken off, and the rear cat had carbon deposits to the point the car wouldnt rev up because it was too clogged. Towed it home and put the new parts on in my garage.

    1. Hey Mike,

      Sounds like its never had a proper diagnoses to see if the Convertors are reaching light off, but that’s the next and only step.


  5. Good morning,

    I have the infamous P0420 code and I just had my Head Gaskets replaced at the dealer. I just read your article. Is it possible that my car may need to relearn running temperature and exhaust leaks and such in order to correct itself? This is my first Subaru. It is a 2010 Forester none turbo.

    1. Hello David,

      My best suggestion is to drive it hard with premium fuel. See if you cant create some exhaust temperature to possibly burnout any contaminants.


  6. Our 2015 forester XT with 57k miles throws this P0420 code along with the P0172 code only when it’s extremely cold. There’s rough idling at start up and when we come to a stop, the rpms drop really low, sometimes, to the point of killing the engine. We only ever use 93 octane. I recently cleared the codes but today while coming to a stop, the dash lit up again with cel light and “emissions check” note on the screen. Would replacing the secondary O2 sensor do anything? I also have a k&n drop in air filter too. May that be the problem? Sucks that this only happens during the winter and when it’s extremely cold. Runs like a champ for the rest of the year. Thank you.

    1. Hey Meng,

      The P0172 needs to be diagnosed and addressed and it just should not be driven until it has been I am afraid. The P0420 may be related to what ever is causing the P0172, but I cant know that until I diagnosed. Even though the symptoms are only really showing up in extreme weather, it would also be true that the component at fault might be robbing you of performance all year long, and that left alone, over time will negatively affect the longevity of the engine and emissions control devices.

      Id just get it sorted out, that Subie has a lot of life left if you want it to.


  7. Hello,

    I’ve just bought a used 2009 Subaru Impreza 2.5i
    Today driving to pick up my kids sudently a solid check engine light, solid traction control light, and flashing cruise light came on.
    I went to part source and had it scanned and a code came back as PO420..
    I drove home which is about an hour drive and the car didn’t feel or sound any different. No change what’s so ever.
    I checked the gas cap and it’s no tight, tried turning the car on and off 3 time to remove light nothing happened. (Suggestions I found on forums)
    Prior to I put cleaner is a full take of gas when I bought the car, still running in that same tank of gas with cleaner.
    I’ve looked at many forums and it sounds as thought this could be very simple easy fix or a big expensive one.
    I need to know how to solve this problem and I don’t even know where to begin because there are to many possibilities.

    1. Hello Jessica,

      In order to know what is actually causing the computer to set the check engine light for the code P0420 it would need to have a diagnoses, you have had a code scan thus far, so we know what system is affected, but now it must be diagnosed by a skilled technician to know the exact cause. It could have a failed Catalyst or a rear 02 sensor, or both.

      A gas cap being loose would never cause a PO420, it could set a P0457 or P0440 however.

      Sorry I cant offer more


  8. Hello,

    I’ve got a 2008 Outback that had a new engine block put in at 77k because the dealership snapped the head bolts off when trying to replace a cracked head gasket. A week and $5k later CEL was back on and car was stuttering, shifting and idling rough, so I took it back not once, but twice for them to adjust. That was early June of 2016.

    Lately, the CEL will come for a few days and then go off a few days. I haven’t been able to pinpoint what precipitates CEL illuminating.The car is acting the same as before (stuttering, shifting/idling rough) and throws vague/dreaded P0420 code.

    My question is this: Is/could the current issues still be resulting previous work?

    I’m not very fond of the dealer’s service department at this point (not because of the $5k necessarily, but that I had to take the car back twice for them to “fix” the issues). My confidence in them is shaky.

    Any advice is appreciated. I plan on putting another 70k on this car, so I’m not opposed to necessary repairs/maintenance no matter the cost.

    Thanks much!

    1. Hi Keri,

      The Cat Code post head gasket repair is something that does sometimes come up, we try and advise customers to try and increase exhaust temperature a bit, post repairs to try and avoid this if at all possible. I have described some of what can go on the best I can in this and other posts here on the website.

      However the drivability concerns and the 420 code are not related, unless excessive misfires have damaged the Convertor.

      Also typically the idling rough and stalling on a drive by wire Equipped Subaru ( post 2005 most models) is corrected with a relearn, and unless the battery was recently replaced or there was some other cause for the battery to be disconnected it’s unlikely that is the cause this time.

      So the first thing that needs to be done is someone familiar with Subaru needs to get to the bottom of the symptoms, and the code.

      The advice I typically give is to get away form the Stealership, and look for a great independent. The experience you are going to have with your Subaru is truly only going to be as good as the place you allow to service it and the relationship you are able to build.

      Hope that helps


  9. Love your site, and I too wish you were closer to Utah. I have an 05 Subaru Legacy Outback coming up with a P0420 code. It’s a California emission vehicle, but I have tried two different non-California catalytic converters, and both have failed. I called the dealer, and they say the car has already had the ECU update. Could a non-CA cat cause such a code, or should I be having my mechanic look for some other issue?


    1. Hello Dave,

      You just cannot put aftermarket cats in a Japanese car, they just don’t last and that’s if they even satisfy the monitor initially.

      Are you also using non OE 02 sensors? That can have an affect as well.


  10. Hi There,

    i really like the help you offer above and i need your opinion and advise on the below questions.

    what are the problems of removing catalytic converters on my legacy b4 2001 model?
    Is a turbo timer really necessary for the same vehicle?


  11. Hello Justin

    Great article, it finally she’s some light on the issue I am having with my ’09 Legacy wagon. CEL popping on/off last while. Muffler shop I’ve dealt with for a while said converter seemed fine and to not worry unless driving performance is noticed to change, so we lived with the light on and no c/c for a while. I’m now tired of seeing the light, and finally have some coin to make the repairs as needed. My biggest confusion is which cat conv is the problem. The muffler shop says there are 4 cat conv (2 at engine, two inline mid exhaust). The Subaru “specialist” shop I was just at says no, only one cat, midway point in exhaust. Reading your article, noting that converters have been moved closer to the engine, I’m inclined to think the muffler shop tech knows more about the system than the Subaru ‘specialist’ (independent shop, not a dealer shop). P0420 code when I looked it up is ‘bank 1’. Is this indicating it is the cat conv at engine end that is reading below threshold.? Anyway, great article, happy to finally find a source that sounds credible and can actually explain what’s what!


    1. Hello Adam,

      I like you have also experience the cat converter which as been a problem since owning the car. I have taken my car to the dealer 3x and 2 x they reprogrammed it and the third time they blame it on needing a tune up. Like yourself I decided to live with it since it wasn’t imparing my driving.

  12. Hi Justin.
    I have a 2009 Outback here in NC. My front CV Boots on both sides need replacing. Axles look fine on both sides. My Mechanic wants to change out the whole thing. If just the boots are bad – at least that I can see, is there any harm in changing just these and leaving the axles “as is”?

    Many thanks. Great website.

    1. Hi John,

      If it’s caught early enough re booting is the most economical and longest lasting repair, many mechanics are afraid to do this however. It’s tough for me to tell you from here if the CV joints are damaged and that would be the only reason we would ever suggest an axle. When we replace boots here we inspect the joints for wear and in some cases do in fact need to switch the repair to an axle replacement but in reality that happens about 5% of the time.


  13. Hi Justin
    MAYBE you can assist me here..i have a 2011 forester diesel turbo.
    i have two light on dash vehicle dynamics control warning light and a emissions control unit malfunction engine control system light on dash.
    a Technician carried out a diagnostic procedure as per workshop manual checked resistance between relay and connectors which led to finding a blown fuse.20A Green.
    and suspected a one time voltage surge.
    was all ok for the day though the following morning on cold start the light re-appeared and the fuse has blown again.
    maybe you can assist me to understand what may be causing this fault…any suggestion and i thank you for your time reading my concerns..
    kind regards


    1. HI Tim,

      I wish we had Turbo Diesel Foresters here, but we just don’t however.

      Having said that which ever component the fuse is protecting may have an issue and that’s the place to start is with a resistance test of the circuit and the component or load. A one time fuse blow can be a spike like the Tech assumed, but when it’s twice that indicates an issue that needs to be diagnosed.


  14. Hello Justin,

    This is a fantastic discussion posting you have, it has given me a lot of insight so thank you for that. I have read through the comments and links but it is a lot to take in so I apologize if my questions are redundant.

    When I purchased my 05 Outback 2 years ago (then 125k, now 144k) the CEL was already on with an error code specific to the front O2 sensor (P0051 I think?). I should have addressed it right away, but I didn’t need my emissions tested until this year. After I changed the sensor, that code went away and the P0420 came up. This code was not appearing before. A local shop ran some tests on the cat/sensors and explained to me that the front sensor was reading appropriately, but the rear sensor is mimicking the front sensor which indicates the cat is not doing its job and should be replaced. Like you, they strongly suggest an OE Subaru part for it to last.

    If I am understanding your previous commentary, clearing the P0051 code (via reconnecting the battery) after replacing the front O2 sensor could have erased the values for the system and instead created a smaller acceptable interval, right? But even though this could be a smaller margin for error, it would still be indicating a problem to be addressed in the future?

    Do you think it is possible that in replacing the front sensor I managed to contaminate the cat somehow?

    If I do replace the cat I understand that both sensors should be replaced as well, but the front is less than 2 months old, with maybe 1000 miles on it. Would it be unwise to keep this sensor for a new cat?

    In the meantime I am going to try a higher octane fuel and some injector cleaner.

    Thank you for your time,

  15. Hello Justin. I have a 2008 Outback with approx. 83000 miles on it and I get the dreaded P0420 code. I have changed both O2 sensors. I even did the spirited drive as you recommended. This is my normal driving style as the wife’s is at a normal driving range. I don’t dispute that the catalytic converter probably needs replaced. What I do question is what are the actual conditions that throw a P0420 code (ie. throttle position, coolant temp, air flow in relation to one or another….). When I drive the car for a week it will throw the code (spirited driving). I reset the code and let the wife drive the car (normal driving) for a week and it will not throw the code. When I drive like my wife drives it will not throw the code. Can you please explain? Thank you for any information on this subject.

    1. Hi Kerry,

      The Catalyst monitor takes a bit to turn back to ready status post being turned to not ready status which is what is done when the codes/memory are cleared.

      Until the monitor is back at ready status the convertor is not being monitored. Ambient temp vs ECT temp at start up combined with drive time at certain speeds are what ultimately trigger the monitor, when we perform a drive cycle post code clear the catalyst and Evap monitors are the last two to come back online, and sometimes can take days or weeks.

      Once its set to ready status before the convertor is monitored it has to be up to op temp and driven at speed for a length of time, it’s not minutes it’s frames of time for which conditions are present

      Was the WVM 23 service campaign performed on your 2008 Outback? That’s the place to start.


  16. I have a 2006 Subaru impreza and live in MA. I got the p0420. Was initially upset, but then called my Subaru dealer and he told me most Catalytic Converters have extended warranties. I have 83,938 miles on my car. My warranty goes to 84,000 or 7/31/14, needless to say i am relieved!!

  17. Hi Justin, I would appeciate your comment on my 2010 Outback. Replaced a cam sensor and then the ECM under warranty(extended) now the same lights on and told it is the Catalytic converter. Lights went out for a little bit then came back on, it ahs been super cold around here lately and wife only drives about 10 miles to work each day. What questions should I ask befroe replacing the Cat Conv? Thanks

    1. Hi Tim,

      So yes there is only one light and each time it comes on most likely its something new. I know that’s confusing at times but Welcome to the US government.

      Id drive the vehicle in a way that builds up a little heat before replacing the convertor. It may very well be that the issue is a weak convertor that is being affected by the colder temperatures only.


      1. Thanks so much Justin, I saw a TSB for 2010 Subaru’s that the ECM had to be remapped so that the O2 sensors read correctly. Is that accurate and if it is a new ECM should I ask if it has been calibrated? It is a PZEV vehicle sold and driven in Pennsylvania. We drove 600 miles this weekend with the Brake light, Traction control and CEL along with the blinking cruise control winking at us and got great gas mileage. Not psyched to pay 950 to replace the convertor. Should I make sure they checked the O2 sensors, A/F fuel ratio, ECT, MAP before I allow them to replace it? Shouldn’t they have thought of all that before recommending to replace?

        1. Hello Tim,

          Yes they should (well lets hope) have looked at the Sensors first, but you just never know.

          Also the Warranty is 8 years 80k on a converter are you over that mileage?


          1. Sadly yes by 5k I told them I am waiting to replace it, have another mechanic who I want to take it to. Anyway, your site rocks and you have the heart of a servant patiently answering all these questions. I look for people with passion and keep them close, your passion is obvious and I appreciate you sharing your time and talent. Rare these days!
            Kind regards,

          2. Wow Tim,

            That’s sucks.

            Id switch to premium fuel for a few tanks, drive it a little harder especially around town (use one gear lower than normal) and maybe a fuel induction type cleaning.

            Because it seems to only show up during the Colder days, Id say its still has some life left, and if it was here that’s what we would suggest.


      2. Hi Tim,
        I have a 2005 Subaru Legacy Wagon Limited and I get code P0420. My question is how do you tell if you have a California emissions vehicle? The car was purchased in Boston, MA and then driven to CA. So I am wondering if I use a non CA converter if my code will be cleared.


        1. No idea who Tim is.

          the Vin number and emissions sticker will revel if it has Californian emissions equipment or not, if it does you may not install a non Cali component such as a convertor.


  18. I have a 2001 Subaru Impreza and I got a P0420 code.
    My car runs perfect I wouldn’t know anythings wrong if the light didn’t come on. I’m still getting the same gas mileage. However, my cruise control isn’t working. Is this an indication of an oxygen sensor? I read somebody else had the same issue. If you have an idea about this I would really appreciate some help

  19. 2000 Subaru Legacy L Wgn 172,500 miles

    I have a recurring CEL code of P0420. Its been happening for about 3 years. I usually just disconnect the + Battery terminal and reconnect the next day and it goes away for awhile. I’d like to get the issue diagnosed properly and was wondering what it would cost for the initial check by AWD Service? I’ve been told by the dealer it is the CAT. Converter etc.


  20. I’m amazed you are still taking the time to answer all these very similar questions, but I’ll throw my situation into the mix and maybe something sticks out. ’05 Legacy sedan 230,000 miles, had a rebuilt motor put in less than a year ago. Throwing the 420 code as well as P0335. Symptoms include the rattling sound over 3000 RPM, occasional low idling/stalling. The biggest pain for me is warm starting issues. If I drive it until it gets to full temp and turn it off she won’t start back up for 15-30 minutes, almost as if it’s flooded, turns over fine, just won’t fire up. My independent Subie guy has access to some used Cats for about $500 installed, but I’m concerned that’s not the only issue. He looked at the sensors and thought the crank shaft sensor was loose but the code came back. Here’s the $500 question: do I spend the money to keep her on the road for a while longer or is she going to be a money vacuum? Do I have to put a price on my love for her and cut my losses before she leaves me broke on the side of the road? Should I have asked for a Prenup.? Hope she doesn’t find out I’ve been talking about her like this. Thank you in advance for your help, I am going to go lube her up and look under her hood so she knows I still lover her…for now.


    1. Hello Adrtian,

      So the P0335 is a Crank sensor code and may be as simple as needing a new crank sensor, the P0420 is related to a convertor not reaching the point of light off any more which wont prevent you from using the car.

      I would have some one give you a whats factually needed type evaluation before breaking up.


  21. Hi Justin,

    I can’t believe you are still answering questions about this code, thanks!

    I just purchased a used 2000 forester with 240k miles, the PO said the head gasket was changed as well as other maintenance but they didn’t have any paper work. Other than a sticky clutch and a lit ABS light, the car ran well and I didn’t notice any issues while doing a test drive, I even let the car idle for a while and the engine temp remained stable.

    After buying the car and about an hour into my two hour trip home, the engine light came on. I noticed no difference in the way the car sounded or ran. At the next stop I popped the hood and noticed a severely leaking battery. I replaced it when I got home and the CEL went away. The next day the CEL came on within about 5 minutes of driving. I checked the code and it was the P0420. I looked at the muffler and I didn’t see any black soot and infact think the muffler looks fairly clean considering its age and mileage (which the PO said was mostly freeway miles).

    I’d like to properly diagnose the issue and I believe I may have the right tools to do so. I have an innovate LM2 wideband sensor, can I use it to get the information necessary to better know whether or not I need a new CAT or O2 sensors?

    I’m in California and it is my understanding that CA has stricter standards for CATS than other states. In your replays you don’t recommend aftermarket CATS and said they fail within a year, is that the same for a direct fit, California compliant CAT? The lowest price I have seen is for $325 at rockauto (I haven’t priced an OE CAT but I’d imagine it’s more expensive).

    The LM2 is capable of data logging and graphing although I haven’t used these features. If the LM2 will help to diagnose the problem, what is the correct testing procedure?

    Thanks again! I really appreciate your help!

    1. Hello Ivwshane,

      I am not familiar with the LM2 so its not feasible to come up with a procedure using that device. If you look at the graphs you will see the pattern from a faulty Cat or one that is good but not up to temperature yet as well as what the pattern looks like when the convertor is functioning okay.

      The data generated is only as good as the sensor, so the sensor must be tested first. From there use the graph as a example of what to look for.

      It took me about 5 years post education to be a good enough technician to really be able to accurately diagnose these type of conditions and even then sometimes more experience may generate even better results such as a better understanding of cause and effect which can lead to fault prevention. Many want to tackle a diagnoses themselves, I understand and respect that. Its just difficult to try and spell out a procedure for you. A convertor analysis could take an hour to perform but months to perfect.

      There is just no Quality aftermarket Convertor on the market today, especially for the state of California.


      1. Thanks for the info. I guess I’ll look into testing the O2 sensors first. Do I need to test all of them or just the rear one?

        To get an OE CAT do I get it from a subaru dealer or is there a place online I can go? Do places that carry the correct CAT charge the same for it?

        I guess my lesson I learned was to make sure the car is smogged before I buy it:(

        1. All sensors should be tested and replaced as needed.

          I have no idea if any one sells a factory Subaru Convertor “online”. It would need to come from someone like us or a Subaru Dealer.


      2. Well, unfortunately the car died before I could even look at it:(

        On the way home from work it had some hesitation that at first I thought was caused by me not shifting correctly however a few miles down the freeway while going 75 the car started losing power and surging. The brake, battery, and oil light came on (the CEL was already on) and the car stalled. I managed to start it again and noticed it was surging while idling.

        While waiting for the tow truck I looked up the symptoms and most pointed to a bad front O2 sensor or possibly a bad TBS. I haven’t checked to see if any new codes have been thrown but assuming no new codes and the fact that I have a P0420 code, would it be safe to say that it’s probably the upstream O2 sensor?

        Or do you not have info info to make an educated guess (I won’t hold you to any answer you give me as I understand there are a ton of variables)?

        Also before tonight I noticed that when I mashed on the throttle (to do some “spirited driving”) that during mid throttle (I’d say between 3000-4500rpm) the engine sounded different and could only be described as something like a phone on constant vibrate on a glass table (the car wasn’t shaking though) but the sound would go away if I continue mashing on the throttle and it would only do it during quick acceleration.

        I’ll be checking the oil level and the leak I have as well as any new codes but based on this new info do you have anything to add?


        1. Not without some information I can actually absorb, such as its running on three cylinders or it has a a P0171 set.

          The 420 code and how it is running will have no correlation 99.9% of the time.

          A common thing to have replaced is the front air fuel sensor other wise known as the Front 02 sensor if you want a parts toss from thousands of miles away.


          1. I checked the codes today, no other codes besides the P0420. I cleared the code and started the car, it ran for a few seconds and then stalled.

            I’ll try a new O2 sensor.


  22. Hi Justin,

    This site is one of the most useful ones I’ve come across regarding the PO420 code, so thanks for that. However, I haven’t seen anything here (or on other sites) yet with my issue. My ’05 Impreza with less than 70k km on it threw the code a few months back without any other error codes. I had the code cleared and it came back after a drive cycle. Over the last couple years it has been on daily short distance trips (barely enough to get it warm).Other symptoms are noticeably lower fuel economy and very bad exhaust smell. Does this help determine if its the cat or something less expensive to fix (rear O2 sensor)? Also, using the Torque smartphone app (I hope you’re familiar with this), what display should I be looking at to check the rear o2 sensor waveform?

    Thanks very much!

    1. Hello Hank,

      Thanks for the feedback. I have not used the Torque app as I have the factory Scan tool but the pictures in the article do show what you need to be looking for when graphing data from the rear 02 sensor when trying to determine if the converter is reaching light off.

      “Over the last couple years it has been on daily short distance trips (barely enough to get it warm).Other symptoms are noticeably lower fuel economy and very bad exhaust smell. Does this help determine if its the cat or something less expensive to fix”

      So the information I pasted above from your post.

      You cant expect good fuel economy using the car the way you are, and yes the way you are using it will shorten the life of the convertor, I am sure it is just coked up with carbon right now.

      So it doesn’t help me determine whats wrong but puts the odds in favor of the convertor being restricted with carbon and not reaching temperature.


  23. Hello, very informative site – thank you for taking the time to share your knowledge and answer questions. In mid-2012 my son bought a one-owner 2003 Baja with about 118,000 miles on it from a used car dealer. At the time of purchase we took it to a Subaru dealership of our own choosing and paid for a multi-point inspection (the same one they do for certified pre-owned cars). That inspection indicated there were no concerns, other than a suggestion to replace the timing belt, which has been done. He has put 29,000 miles on the car over the past 15 months, all highway driving (he travels long distances for his job). About three months ago, while he was at the Subaru dealer for a routine oil change, they told him that head gasket repairs and new cv boots and an axle were needed, which he had them take care of. This was a pretty good chunk of change, so I’m sure he was hoping it was good to go for awhile, but now this morning the Check Engine Light came on. He took it in to the Subaru dealer and they say it is a p0240 code and it needs a new catalytic converter at a cost of $1800. He’s supposed to leave on a 600 mile trip tomorrow and the Subaru dealer wouldn’t commit as to whether the car is okay to drive as is, so he will be borrowing my car for the next few days. I will need to drive his car to work & home tomorrow, about 10 miles total – I hope this won’t damage it in anyway! Then the car will sit in my driveway until he gets back from his trip and he decides on how to proceed. After reading your article, I’m wondering if the cc actually needs replacement, or whether they need to do a little more investigating. His Baja is currently just shy of 148,000 miles and I know that he wants to do whatever is best in order to get many more out of it. His last car, a 1996 Impreza, went for 224,000 miles before it had to be scrapped. Thanks for any insights you can offer!

    1. Hello Raina,

      The vehicle is completely safe to drive with a P0420 set, however if anything new was to come up that could damage anything, you would have no way of knowing as the check engine light would already be on.

      That’s why no one really wants to tell you if you should drive it, a good idea is having your own code scanner and checking it your self here and there until you can budget for repairs if you choose to do them.

      Hope that helps.


      1. Thank you Justin, that is good to know! Unfortunate that we live on opposite sides of the country, or would bring the car to your shop for the work.

  24. Hello,

    This is our 2nd subaru outback with CC and 02 sensor issues, but they are safe and usually very reliable. This one is a 2008 and just over two weeks ago was at a subaru dealer in St. Louis to replace the left or right front axle. We also asked they do a 100,000 mile check for issues. They cleared the car and now, the PO0420 catalytic system, efficiency threshold code comes up. My question is, would checking the CC, 02 sensors, etc. typically be part of a 100,000 mile check up?

    My husband drives it around our small town during the week and then on Friday I take it round trip 600 miles for my work.

    Thanks for the info and guidance.


  25. I have a 2007 Forester, base model. Driving at a high speed. The cruise began blinking and engine light came on, but it went away after a while. Failed inspection recently with a P0420 code. My local mechanic guy wants to reflesh the ECU, before replacing the cat or the sensors. Good idea?

    1. Hello Brian,

      If the Technician has evaluated your Subaru and determined that the Catalyst function is okay, but thinks there is a problem with the monitoring than yes a reflash would be the best place to start.

      Hope that helps


  26. Thank you for the information. After reading your explanations I end up with more questions than I started with. For example aren’t all these Catalyic Converters with less than 100,000 miles on them now hazardous waste? Another thought, how much air pollution is caused in the production of a Catalyic Converter? Rant over.
    Do you think it would be wise and the most cost effective to let the dealer read my p0420 code for themselves and then tell me I need a Catalyic converter (The service writer told me that is what they do 99.9% of the time with the p0420 code). My car is a 2.5 06 Outback with 85,000 miles. Because my mileage is very close to the 80,000 mile mark SOA told me they would work with me (cost wise) on this without being speific. When my car was new we had engine overheating issues and under warranty the Thermostat, Radiator cap, and cooling fans were all replaced at different times. In other words my engine was running hot for the first 20,000 miles or so. Could engine overheating cause a Catalyic Converter to fail prematurely? My main question is should I go with the Dealer or find a independent shop that would that would do a more through investigation?
    Thank you

    1. Hello Ken,

      Catalytic Converters are recyclable and just like any other recyclable material including engine oil and coolant is only hazardous if not handled properly. As far as what the service writer told you there are some dealers that will scan a code and give you an estimate without doing any real type of diagnosis. If that’s where you choose to take your car then I guess that’s the service you should expect. Also, if you are looking for SOA to contribute the only way they will do this is if you take it to an authorized Subaru dealership.

      Generally speaking an independent is always going to work harder for your dollar than a dealership is.

      Hope that helps.


  27. 06 Outback manual trans. 85,000 miles. My car too has the p0420 code. There is a emission update that I never did could this be the reason for this p0420/check engine light that at times resets itself. Wish I would have looked into this 5,000 miles ago.

    1. Hello Ken,

      Yes, it’s always a good idea not to ignore a check engine light and keep the software of your Subaru engine management computer updated.

      Hope that helps.


  28. I too have an intermittent CEL on my 2006 Outback, 148k miles.I had the code checked,P0420. A local garage said the c at is wearing out and has a rattle. O2 sensors are Ok for now. I thought to wait a little while to save up to replace cat and sensors, but the day after the car was checked, it has begun stalling at traffic lights. The engine will die if I don’t put it in neutral and give it gas.Any ideas on what is going on? I really appreciate your advice. THANKS.

    1. Hello Tammy,

      Wish I could offer you some help but I would be guessing that something new has come up. Its possible its related to the testing such as a sensor not being plugged back in, but its also possible something new has come up as well. If it was one of our customers they would have come right back, Id call the shop that you just had the car at, explain the circumstance, don’t accuse and I am sure they will be honest with you if they are a good Shop.


  29. Hello, I have a 2001 Subaru Forester with 111,000 plus miles on it’s 2.5 liter engine. I have been driving it around for over a year. I bought it used so, I have no reference to what may or may not have gone on with for the previous owner. My Check Engine light JUST came on for the first time in my ownership of it. It does not seem to be running any different. I checked my gas cap but, have not driven it around to see if it resets. I also noticed that my Coolant Reservoir was empty. I do not know if that has anything to do with it but, I did add some 50/50 coolant. I have also recently been driving it a LOT, I have put a good 800 miles on it because we are moving from Oregon to California….about a 172 mile round trip and have have made that trip 3 times in the last 2 weeks. Any help or suggestions would be GREATLY appreciated. Thank You.

    Jacob (poor and worried)

    1. Hello Jacob,

      Because you are poor and worried the first thing you need to do is head to a parts store that will scan the car for free. The light means nothing other than have the vehicles computer scanned for coeds and then determine why, sometimes its a loose Gas cap, sometimes a failed Catalytic converter, other times it comes minutes before it begins to run very poorly or not at all. Without a code I have no idea what to try and suggest you check, while its always good to check the car fluids, the check engine light is not generally ever going to come on for low fluids.

      You can read here for more information about how the system works, and its always a good idea to buy your own code scanner.

      Or this

      Or this

      Hope that helps


  30. hi there i have a subaro wrx 1998 and the check engine starts to come on and after a while it goes and after a while it starts to to come on and when it does tat the RPM goes lower than 0.8 and it turns the car off and if i was driving and the chek engin turned on when i am pressing on gas it feel like iam releasing my leg on gas but i am not releasing my leg on it and it comes back again but with defrent performance and after a while it starts to bring out a white smoke from rear (not so white) and with a bad smile, but if i turend the car off and starts again after like 1 or 2 hours nothing is hapening unless i drive like 6 miles its comes again, but now the chek engine is on when i start the car and the RPM is below the 0.7 and i cant drive it because the whole thing i told u about is happening when i does, plz help me knowing whats the problem and i will be greatfull.

  31. Justin, thanks for sharing your expertise on this site. It really helps to have a knowledgable Subaru mechanic give advice like your do. You’re doing a great service to the Subaru owner community. I now am on my fourth Subaru Outback/Legacy in a row.

    My current Outback is a 2006 Limited with @150,000 miles on it. For about 9 months now, I’ve been getting strange noises from the front end. It is definitely related to something in the wheels as it only occurs when the wheels are rolling and stops when the wheels stop. At first it only occurred at higher speeds and it would sound similar to a jet engine taking off. Later it starting changing in tone and frequency to a variety of sounds including a scraping type of sound. It also started occurring at slow speeds (driving around a parking lot).

    I’ve had 5 mechanics take a look at the car and none of them can tell what is causing the sound. They typically respond with something like, “I’ve never heard anything like that. I have no idea what is causing it.” They’ve checked all the obvious things–brake rotors & pads, tire noise (a new set of tires did not change the noise), wheel housings and mud flaps contacting the wheel, etc..

    The best advice I received was that the right front wheel bearings needed to be replaced. So I had both front bearings replaced. Unfortunately this did not work either. And ever since that repair, from time to time the CEL comes on and the cruise light blinks. When I test with an OBD reader, I get the P0420 code. Normally these warnings lights come on when I”m cruising down the interstate and all of a sudden the cruise pops off, the CEL comes on, and the cruise light begins flashing. So I think these lights are coming on because of something wrong the speed sensing controls rather than something wrong with the engine or emissions systems.

    The first time this happened, I took it back to the shop that replaced the wheel bearings and they found that the speed sensor on one of the wheels had been damaged during wheel bearing installation and they replaced that speed sensor. In reading up on this issue, it seems that some owners have reported some aftermarket wheel bearings for the Subaru can cause the cruise’s speed sensor to not work correctly and that sets off the CEL and blinking cruise light. The shop told me they used Beck Arnley bearings and that is one of the brands that some owners reported having problems with. Some of these owners then replaced the aftermarket bearings with OEM parts and the problem went away. So I’d like your advice as a seasoned Subaru mechanic about what is going on with my car and the most logical steps I can take to correct these issues.

    What do you think is causing these sounds from the front end?

    Should I replace the aftermarket front bearings with OEM bearings?

    Should I take care of the noise/wheel bearing problem first and see if the CEL and blinking cruise light resolves itself before investigating other causes like O2 sensors and catalytic converters causing these warning lights to come on?

    Thanks in advance for any advice you care to give.


  32. Justin, thanks for sharing your expertise on this site. It really helps to have a knowledgable Subaru mechanic give advice like your do. You’re doing a great service to the Subaru owner community. I now am on my fourth Subaru Outback/Legacy in a row.

    My current Outback is a 2006 Limited with @150,000 miles on it. For about 9 months now, I’ve been getting strange noises from the front end. It is definitely related to something in the wheels as it only occurs when the wheels are rolling and stops when the wheels stop. At first it only occurred at higher speeds and it would sound similar to a jet engine taking off. Later it starting changing in tone and frequency to a variety of sounds including a scraping type of sound. It also started occurring at slow speeds (driving around a parking lot).

    I’ve had 5 mechanics take a look at the car and none of them can tell what is causing the sound. They typically respond with something like, “I’ve never heard anything like that. I have no idea what is causing it.” They’ve checked all the obvious things–brake rotors & pads, tire noise (a new set of tires did not change the noise), wheel housings and mud flaps contacting the wheel, etc..

    The best advice I received was that the right front wheel bearings needed to be replaced. So I had both front bearings replaced. Unfortunately this did not work either. And ever since that repair, from time to time the CEL comes on and the cruise light blinks. When I test with an OBD reader, I get the P0420 code.

    The first time this happened, I took it back to the shop that replaced the wheel bearings and they found that the speed sensor on one of the wheels had been damaged during wheel bearing replacement and they replaced that speed sensor. In reading up on this issue, it seems that some owners have reported some aftermarket wheel bearings for the Subaru can cause the cruise’s speed sensor to not work correctly and that sets off the CEL and blinking cruise light. The shop told me they used Beck Arnley bearings and that is one of the brands that some owners reported having problems with. Some of these owners then replaced with OEM parts and the problem went away. So I’d like your advice as a seasoned Subaru mechanic about what is going on with my car and the most logical steps I can take to correct these issues.

    What do you think is causing these sounds from the front end?

    Should I replace the aftermarket front bearings with OEM bearings?

    Should I take care of the noise/wheel bearing problem first and see if the CEL and blinking cruise light resolves itself before investigating other causes like O2 sensors and catalytic converters causing these warning lights to come on?

    Thanks in advance for any advice you care to give.


    1. Hello Larry,

      Thanks for the Kind words.

      I would in fact start with having Subaru OEM type bearings re-installed, if the bearings themselves were in fact the original cause and we installed suspect bearings the diagnoses or theory may have been correct but the correction suspect.

      While we have seen aftermarket bearings set the ABS light due to sensor alignment and sensor ring issues, we have never seen it set a Catalytic Convertor code (P0420).

      To the cause of the noise, has anyone evaluated any of the seals in the front rotating masses for dryness and noise?

      Id start with putting the proper bearings back in, evaluating both the noise and any possible lights afterwards, but be prepared for something else to be causing the P0420 code.

      Hope that helps


      1. Justin, Thanks for getting back to me. The day after I posted that I came to a sudden stop when a green light turned to red and my Suby violently shook as I came to a stop. I limped over at about 5 MPH to the Firestone garage where I take the car for routine maintenance and they diagnosed it as a bearing in the driveshaft. I then had it towed to a local transmission shop where it is awaiting a new driveshaft from Subaru. Should be done in the next day or so. Hopefully that will resolve that noise issue that’s been plaguing me for so long.

        When I get it back, I’ll get those front wheel bearings changed to OEM parts and I believe that should take care of the CEL and flashing cruise light. Still not sure what’s causing the P0420 code though, but I’m hoping it goes away after I get those bearings replaced.

        Thanks for the advice.


  33. Justin,
    We purchased a used 2002 Outback wagon recently. CE light came on yesterday. My son is an auto tech, and scanned to find the dreaded P0240 code. Your article says a possible cause is “sometimes its just a small exhaust leak”. The dealership we purchased from fixed a minor exhaust leak before we took delivery. Any chance a remaining leak could be the root cause of this code?

    Second, we also have a 2007 Outback wagon, and earlier this year, had the head gaskets replaced (under drive train warranty, yay!). Had the CE light come on, took to the same Subaru dealership (Wagner’s in Fairborn, OH – great place) and the diagnosis was rear O2 sensor. Had it replaced, and light is out. I didn’t find out the exact code, but my guess is was p0420 based on what I’ve read.

    What say you to these issues?


    1. Hello Tracy,

      Its possible there is still an Exhaust leak, but I wouldn’t know that to be accurate unless I see it. Has your son had any luck finding an Exhaust leak since he is a Tech? If you are not able to locate one, than that is most likely ruled out as a possibility. Maybe just drive it for a bit and see how it goes?

      Most likely the rear 02 was set for a rear 02 sensor code on the 2007 as its fairly common.

      Hope that helps


  34. Hi Justin,
    I love the advice you give. Quick question for you, been dealing with an on again off again P0420 code for about 5 months. Now it’s permanently on. I also hear a rattle up front when starting up the car and at high revs. Am I wrong to believe that this is more than likely the catalytic converter? It’s a 2008 subaru legacy 2.5i limited with 99800 miles. Also, I’m looking at replacement cat’s online and I see on some places that there’s a rear cat, other places it just show’s a front cat that connect’s with the exhause manifold, and in other places there’s what they call direct fit replacements where it looks like a rear and front cat but only the midsection (not up to manifold). Which of these am I looking for if it is the cat that needs replacing?? Thanks a lot bud!

    1. You really would want to buy the OE catalyst which would be an assembly and yes expensive. The issue at hand is the aftermarket ones may not have enough precious metal to function.

      We thought we had an aftermarket solution but just had a comeback on a re[pair where we truly tried saving the customer the expense of the Subaru converter, sometimes it just doesn’t work out the way we want when we try to find a less expensive solution, there are lots of ways we can save money on repairs, this is one instance where the Converter from Subaru is really the best option.


  35. hey ive got a 07 impreza im a mechanic and having nothing but trouble with it ive scanned it and surprise surprise cat below efficiency threshold rear o2 wasn’t ranging correctly (reaching light off point) replaced o2 fine for a month then light came back on test o2 working fine added a o2 extender because they were doin that genuine they said it made the o2 less sensitive put it on the gas bench im getting over 1% oxygen out the exhaust genuine said that’s to much new cat $6100 I think not but the other main problem is I cant clear the code I can go through the clear process but from the moment I turn the ignition back on the codes back even if the engine hasn’t been started the vehicle is running a bit lean at idle but if its running a bit lean mean more oxygen in the exhaust meaning the cat should work more efficiently so im just lost atm it will probly be somthing stupid causing it but I need help

    thank you regards liam

    1. Hi Liam,

      $6100 is way to much $. Need to perform a Catalyst test and try to determine whats wrong.

      Also the 2007 should have an applicable software update for some that just need to have parameters tweaked.


  36. Justin,

    I had the check light come on. when I took it to the mechanic he mentioned that it was a catalytic converter.

    I did not get it replace. I have been using low grade gas. So i decided to switch back to premium which I used to use in the past. After this switch the check engine light disappeared. Then again it came on. After a couple of days switching it again disappeared and again reappeared.

    Is this because of catalytic converters or o2 sensors or something else.


    1. Thats an indication that the combustion temperature with regular unleaded is not hot enough to let the converter get to the point of “light off”

      Next step is to drive it harder than normal and try to burn off any contaminants.


  37. We recently had our 2006 subaru outback in for inspection/emissions. They stated the head gasket was leading and it needed a new cv boot. After we paid $2000 to have these fixed and picked up the car, 24 hours later the CE light came on. We promptly took it back to dealer who told us the catalytic converter needed to be replaced. I find this hard to believe and too much of a coincidance that this occurred following our service. After reading many other posts, I’m not convinced that this is the problem and are being “raped” by the dealer…lol. My outback just turned 100,000 miles and never has the CEL come on since I’ve owned it. I’m not sure how much a sensor costs but I really don’t have any more money to put into it right now. Any ideas?


    1. Hi Melissa,

      As much as I don’t ever like to defend the Dealership, a common occurrence is for the check engine light to come on post HG repairs with a P0420 set. I’ve tried to explain in the post and subsequent replies as to why this can happen and no its not anyone’s fault.

      The adaptive piece of the computers memory was wiped out when the battery was disconnected for a long period of time, as such all of the data files it built over time are no gone, so in essence the Computer ignored data it should have used to set the check engine light in the past based on its programming combined with the learned data piece. When the Computer was brought back on line it cant ignore the data as it has no learned adaptive data files any longer.

      What to do from here? Its possible after some spirited driving the Catalyst function could improve and light goes out, but under stand its a warning of things to come.


      1. Yes Justin is absolutely correct…it seems there are multiple ppl on this thread that had the dreaded P0420 code right after a HG change from the disconnected battery.

        I personally did…

        1. Put in some 94 octane, and some fuel injector cleaner and had that spirited drive on the highway to put some life back into the Cat (as per Justin’s great recommendation!)

        2. My mechanic thought perhaps the wiring on the O2 sensor was a little bit too close to the heated parts of the engine and tinkered it a bit by tying it away from the heat. (he determined that it wasnt a o2 sensor problem)

        I don’t know which one it was…but it worked…no P0420 code for the last 3 weeks….I was really stressed out after forking out 1700 for a HG change and some other sensors that were leaking oil…and then a potential $1500 Cat change…

  38. Hi Justin,

    Have a 2008 Outback 2.5i Pzev and got a P0420 code after a HG change (external coolant leak). The car was originally imported from NY and has been in Ontario since (close to 135,000km/84000miles). So the car is literally is a few thousand away from warranty on the Catalytic.

    After 2 resets and air out the car on the hwy with 91 octane and fuel injector cleaner…the code hasnt popped back up (knock on wood).

    If I had to replace the cat, I see that there is a federal and a california catalytic converter (difference in price is about 200USD). Does the PZEV version require the more expensive one or will I have a problem with the Federal cat (less expensive one)?

    Thanks for reading. Look forward to hearing from you.

  39. Very good advice on P0420. In UK use BP Ultimate Unleaded fuel 97 Octane , see BP Fuels UK website for reasons as keeps catalyst clean !. Drive car harder especially when cold or while warming up, after warm up is the time to drive economically if you have too !
    Best Regards

  40. Hi Justin,

    Another guy from Canada. I have a 2008 Outback 2.5i Pzev with 85,000 miles (imported from NY State new) with a P0420. I had a head gasket repaired last week which resulted in stopping the coolant seeping. The battery was unplugged for 2 days and so the ECM was reset. Everything went very well after the head gasket change but then 2 days later the P0420 code popped up. My mechanic is telling me that both O2 sensors are operational which leaves him to believe that it’s the front Cat.

    Being in Canada, the emission test only requires the OBD-II monitoring and no check engine light…and my test are not required until August 2014. If I was to budget for a front cat…can the front cat be the OEM Federal Standard version or does it have to be the OEM California Standard Cat since my car is PZEV? The difference in price is $300 more it seems.

    Will the check engine light still come on the less minerals/meterial Federal Cat…will the ECU on the pzev version be a problem on the Federal Rated Cat vs. the California one?

    SOA has already declined my warranty on the cat even though my warranty specifically reads PZEV warranty on 150,000 miles that are distributed to NY, and say that it’s not longer warranted since I am registered in Canada. In fact, I am not even part of the recent recall Jan 2013.

    1. Hi Calvin,

      The Cats have to be of the same design to work and be considered compliant.

      The aftermarket cats just don’t last very long and the slightest amount of carbon prevents function.

      Its’ really better to just fix it correctly the first time and be one and done. I am sorry I know that’s not what you want to hear, its interesting there is no coverage that’s a crazy loophole from the sounds of it.


  41. Hi Justin,

    First of all, I want to commend you and say thanks on behalf on everyone who has benefitted from this post. You are still replying to people and seem like a genuine person.

    Now, here is my question. I bought a ’98 Subaru Legacy GT station wagon in Nov 2012. Some weeks later, in early December, CEL came on. We were still in contact with the seller and he came and checked it out and code P0420 came up, but he said that it was nothing to be worried about and turned the light off. It was a salvage title and the seller was a hobbyist mechanic who I believe worked a little bit on the car before selling it to us so I kind of believed him. Well, the light came on again maybe in mid January. I got my oil change at a shop in late February and mechanic that did it said it’s not too serious considering the car was smogged in Nov12 and turned the light off. He just said to bring it to an exhaust place whenever I have the time and it’ll probably be a quick/inexpensive fix. Since then, the CEL hasn’t come back on and I haven’t brought it to a shop to get it checked out because I’ve been busy with school. My dad recommends that I just sell the car, in fear that it may cost us much more than we bargained for. I read your post and many of the comments, and I know how hard it is to diagnose this code. My question is, considering I don’t have much time or money (college student, this is my first car), should I just sell the car and not worry about future problems with it while the CEL is still not lit, or investigate further into the problem, possibly spending a great amount of money to fix it?

    Thanks, JJ

    1. Hello JJ,

      Thanks for the kind words!

      If the rest of the car is in good shape I would never suggest you get rid of it for a failed convertor, you may be able to get many more years of good life out of the car, the converter is an emissions control device and even when the light is on ass a result of the Computer not seeing the data it wants, the car may not be grossly polluting. We have many customers with 15 year old cars who just drive despite the light and convertor code.

      Hope that helps


  42. Great set of articles. I’ve learned more here in the last 20 minutes, not just about emissions, but servicing and testing in general.

    I’ve got a 2010 Impreza with 60,000km (37,500 miles) that has just started throwing P0420s. That’s not very long. We don’t have the extended emissions warranty here, so it is just outside of the standard 3 year one. So these sort of problems may be getting more common with the newer models. Or mine might have another common fault, as you detail.

    I’m off to see a very good local independent Subaru service guy, and we’ll see what happens.


  43. Hi Justin,

    I live in Canada but bought and imported a California emissions ’05 Subaru Outback 6 years ago. Low and behold recently I’ve gotten the dreaded P0420. I spent 3 hours with a shop today who specializes in exhaust systems. In fact, the local Subaru dealership sends many of their customers with P0420s to this shop because of the outrageous costs for the catalytic converters on “Canadian” Subarus (only 2 converters on Outbacks found in Canada, $2600 and $1300 for the two cats). Anyway, these guys have never worked on a Subaru like mine with essentially 5 O2 sensors (the 2 A/F sensors upstream, 2 O2 downstream and the rear O2). Since you may have more familiarity with dealing with my exhaust system, here are the notes from today after a bunch of diagnostics:
    – Bank 1 Below Threshold
    – Short Term Fuel Trim in normal ranges during driving; however at idle hot spikes from -13.3 to +29.7 (normal ranges +/-10)
    -O2 Sensors switching in normal ranges on test drive; air fuel sensors are constant as well
    – We did notice that some of the O2 sensors were flatlining but would switch back on

    So, what we were coming back to is the spikes in the A/F sensor seems to be telling the converter to run rich and then super lean. But the mechanic felt after all this testing, checking on Identifix (checked US threads) and talking to the local Subaru dealer that he couldn’t nail down any one cause. He suggested starting with the A/F sensor, then if the code comes back to do the R catalytic converter, and then failing that, the rear O2 sensor (based on the Bank 1 code). Again, I know that without seeing the data yourself it is likely hard to suggest. Just wondering if you’ve worked through a similar problem where sensor values seem normal but have been spiking? Any insight would be helpful. Also, is there another test the garage should be doing? Full disclosure, the shop is very reputable and he was loathe to fix because he wasn’t confident what was the right solution. Wasn’t like he was trying to take my money.

    Also, thanks for taking the time to help out fellow Subaru drivers. It’s pretty commendable!

    1. Hi Justin,

      First of all you have a great name.

      Very familiar with that converter and sensor configuration.

      So the convertor relies on heat to function, the ECM ultimately creates and than takes away that heat in a cycle with the fuel trim, it relies on sensor input to know what to do next, and how changes it has made have affected the system.

      Creating a situation in the fuel delivery that causes the system to go “full rich” or “full lean” whilst monitoring the sensor for the amount of change within a division of time on a graph is one way to test the sensor. You must know what is a “normal reading” however in order to know if the data shown is acceptable or not.

      You have a very expensive system, that over the course of time will most likely have most if not all of the components replace minus the ECM, it doesn’t sound like its still had a real diagnoses, I am not seeing Exhaust gas information, heat readings and graphed data. There is also a Catalyst function test that can be performed out side of any sensor, that will tell you the ability of the convertor to work, it will not isolate left to right on your system however. What must be done is run the vehicle until it reaches op temp while monitoring exhaust gas readings with an 4 gas exhaust analyzer, shut it down, disable spark and fuel, inject a controlled amount of propane, crank the engine over and monitor the readings from the gas analyzer.

      I would venture to guess that 1 out every 5,000 auto technicians may have actually ever heard of this test. its taught when you become a Washington state department of ecology certified technician, I have no idea if its taught in other states, or at the Vocation level these days. Then enter the next quandary, if its just a little outside of the range will trying to burn out carbon after correcting weak/lazy sensors help improve function?

      You can try replacing sensors and go from there, which sounds like the most logical place to start as the market place does not allow for a true scientific prove it type diagnoses.


  44. Hello,
    I have
    2008 Legacy 2.5i
    106K miles

    Had a major engine service about 4 months ago and now check engine light is on. Noticed MPG drop right after engine work and told dealership twice. Argument was it is winter gas blend and just a coincidence. Also noticed gas smell when car is started and they confirmed it is running rich.
    Here is a scan of the diagnostic that dealership gave me…c%20Subaru.pdf

    -Is diagnostic conclusive that I need a new cat? Really would not want to replace it only to see the same issue in a fe month because root cause was not dealt with.
    -Does cat affect fuel efficiency?
    -Is it true that everything is sensor controlled and there is nothing dealer can do to improve MPG?

    Will appreciate any advice.

    1. If the Rear O2 sensor was tested out side of the data you gave me than yes the Catalyst is not reaching light off and will need to be replaced to satisfy the check engine light and the P0420 code.

      A restricted convertor will negative affect fuel economy. But so will many other deficiencies in other systems.

      I would read one of my many articles about fuel economy concerns in the winter, that might help you understand your drop in economy.

      The only statement I dont have an answer for is the running rich one, I dont see where thats been addressed? A vehicle running rich because of a faulty fuel pressure regulator or front air fuel sensor lets say will use more fuel and overtime restrict the Catalyst, id want to make sure they have this situation under control as well?

      Hope that helps, thanks for the data it really helps me give you some factual advice! Even if its not what you want to hear.


      1. Actually I do not mind your answer. I am considering to keep my subaru another 100k. Since it is payed off, instead of paying car payments for a new one I am setting aside monthly some car funds. This will be covered by that fund.

        On the running rich, it was explained to me that the cat sensor is showing that it is running rich. I objected by saying that it could be running rich for two reasons:
        -cat is not doing its job
        -engine is not doing its job.
        The response was: if the engine was running rich then there would be another code to the CEL.

        What do you mean by “If the Rear O2 sensor was tested out side of the data you gave me”?
        From what I understood they are only going by OBD diagnostics. I suspect they did not do much more than that.

        1. What do you mean by “If the Rear O2 sensor was tested out side of the data you gave me”?
          From what I understood they are only going by OBD diagnostics. I suspect they did not do much more than that.”

          Data from any sensor is only as good as the sensor it self! If you have a heart monitor that states your dead, but your talking the first thing a good practitioner will do is swap out the sensor before pronouncing you dead.

          Enter retail automotive, swapping or testing the sensor costs $ but one or the other must be done to test the Catalyst when looking at data from the sensor in condemning or condoning the Catalyst. The industry or the market place rather does not allow for scientifically approaching the issue with a prove good or prove failed circumstance, instead its not an exact science unless you have the ability or the inclination to go one or several steps further in your diagnoses.

          Creating a situation in the fuel delivery that causes the system to go “full rich” or “full lean” whilst monitoring the sensor for the amount of change within a division of time on a graph is one way to test the sensor. You must know what is a “normal reading” however in order to know if the data shown is acceptable or not.


  45. Hi,
    I have a 2004 UK Impreza STI, just over a year ago I had the CEL light come on and diagnosed as the front O2 sensor which was replaced & fault code reset. Since then I get at random times, can be a couple of days to a couple of weeks before getting the P0420 code up (this will reset straight away eveytime) It tends to come on when you just cruise along at a steady speed. My thinking is that either the cat has fallen apart or faulty rear O2 sensor. Any other thoughts or advice would be appreciated.


  46. Opps, I forgot my third question. Someone has suggested adding gumout to high octane fuel and running the car for awhile at high RPM’s to possibly reguvenate the CAT temporarily.

    Is this a reasonable thing to do? What fuel additive would you suggest, if any?

    1. Hi Paul,

      If the Cat is restricted with Carbon, that carbon will prevent the light off temperature from being realized, if the carbon can be cleaned the Catalyst may return to the light of stage.

      Chemicals and Exhaust Temps can acheive this, you can also remove the Convertor and sensors and try soaking it. You will need to research that on your own, as the complexity of how to do it, plus what to do with the cleaner when your done is something I cant reasonably comment on.

      We at the shop will often do induction services with new sensors followed by driving instructions, Id say it helps 30 percent of the carbon coated convertors if I had to put a number on it.


  47. Justin: I have a P0429 code on my 2005 Subaru Legacy with 113 K miles. I have been doing a good deal of research on the internet on the code with the hope that I might be able to avoid replacing the CAT. Your article is the most helpful that I have found thus far.

    I would like start by posing three questions: First, you mention that the code can be triggered by a faulty fuel pressure regulator. In my case, about a week before the code was tripped, I brought the car to my regular mechanic because we were smelling a fuel leak. He examined the car and found a loose hose to (I think) the fuel pressure regulator or at least something that sounds like that. He was able to get a wrench on the hose clamp to tighten it down. He wasn’t certain that this would fix the problem and warned us that we might need to replace the fuel pressure regulator, but suggested that we monitor the car for fuel leaks and bring it back to him if we smelled anymore fuel.

    I am wondering whether this problem could be related to the P0420 code. If so, how? What, if any, additional trouble shooting should I request on the fuel pressure regulator?

    Second, I installed a Torque Pro App on my android phone and purchased a blue tooth compatable code sender. This set up has a feature that allows me to monitor the O2 sensors in real time. I am interested to see whether my cat reaches a “lights out” condition. How is this test performed–while driving or during idle? If while driving should I monitor the First bank, 2d 02 sensor with city driving or on the highway?

    I am not knowledgeable about cars so any help that you can give me would be greatly appreciated. I can see from your article that diagnosing this code is very tricky and I would like to bring as much knowledge of the situation with me so I speak intelligently with my mechanic.

    1. The light off will be shown by data from the rear o2 sensor.

      I prefer to graph it, but you will be looking for the voltage to stabilize at speed during the light of period, where it should be up and down when not at light off. This will be at op temp only and most likely never at idle.


      1. I was able to graph the 2d O2 sensor. After warm up I ran it while parked at about 3000 rpm. I think I observed a good lights out condition (flat line) but with periodic rapid drop offs down to zero and then steep return to lights out. Does this mean I have a lazy sensor that is going bad and a likely cause of p0420 code?

        1. Paul,

          Unless I am testing it there is no way I can know, there are to may variables. Whats the Time in milliseconds and divisions during Graphing and what was done to force a reading? Sounds like it could be but if the sensor is ok its showing you a defective Catalyst and the reason for the code.


  48. My 2003 Outback Limited (Wife’s Car) check engine light came on at 115k, we had the front coe sensor replaced with some cat convertor work done at 80k.

    Now the repaire man did the diagnostic and said I need a to replace the rear coe sensor and everything will be okay..Car when driving seems sluglish to me even though I don’t dirve it often. Price for sensor & labor $400.00

    Please advise if this is the way to go..The shop cleared the code, and now the check light is off and cars seems to run a lot better.

  49. Hi Justin-

    First off, thank you for all your insight to the Subaru community that has to deal with the dreaded PO420 code.

    I have an 01 Sub Legacy. 160K. HGs replaced at 140K by Subaru.

    At about 157K, I remember hearing a rattle from the bottom of the motor. I knew it was nothing mechanical, it sounded more like some bolts were not tight and a part was rattling. After some research, I assume the rattle was coming from the CAT.

    I am in WY where it gets cold. When the car is warming up in the morning, I smell gas odor, but it goes away when I begin driving.

    CEL came on recently and it’s PO420.

    Cruise Control no longer works.

    Are there some other signs that the CAT has failed? Given the rattling, gas odor, CC inoperable, and the PO420, is it reasonable to assume the CAT needs replacing?

    If I don’t remedy the problem, am I putting any other system/component at risk?

    Thank you for your time.


    1. The Cruise is disabled with a powertrain code present in the ECM.

      The Cat may be in need of replacement, but it could have an issue with a sensor as well, I don’t know without testing it whats wrong, or what to replace. Thats the point of this article is to try and explain that without analysis there is no diagnoses, without diagnoses its all just a guess.

      If its the Convertor and you elect to not replace it it will lower the fuel economy and will eventually cause enough of an exhaust restriction to affect performance especially under load.

      The gas odor is probably a fuel leak, use the search feature and type in fuel smell and have a read.

      Hope that helps


  50. I have a 2005 Outback with the SOHC 2.5. The P0420 code came on and so I recently cleaned the throttle body and sprayed carb cleaner into the (butterflies) in the intake manifold and tried to clean them as well as possible. The light stayed off for a week or two but then came back on and now never stays off for long even when I have the code cleared. Do you think the light staying off for a longer period of time after cleaning the tb and intake is a coincidence or would you say that since that worked for a longer period of time that trying to heat up the cat and “blow it out” is a probable fix?

    1. You don’t ever want to use Carb cleaner on the Throttle Body! There is special intake type cleaner you want to use.

      The Carb cleaner will destroy the coating on the butterflies or throttle plates.

      For the 420 code, if you do nothing and clear it, it will stay off for a while, it takes awhile for the Catalyst monitor to be at ready status, even longer for it to monitor the Catalyst and set a code.

      Without data you dont know if the Catalyst function is improved or not.

      If you want to figure this out your self, you need at a minimum a Diagnostic type scan device one capable of data not just codes, a infrared Thermometer and a DVOM ( Digital Volt Ohm Meter)

      Or you can just try stuff, like 02 sensors, super unleaded, getting the exhaust hot and hope for the best.

      Hope that helps


  51. Hi Justin,
    I have a 2004 Legacy with the 420 code. The cats,o2 sensors, Pcm replaced and reprogrammed, Maf sensor have all been replaced with Subaru parts. Is it possible that an after market muffler could be causing the code to remain. At one time I had a running to lean code but it has cleared its self.

    Thank you for any info I can get

    1. Hello Roy,

      Is it possible the aftermarket muffler is leaking ?

      Is the Catalytic Convertor a Factory Subaru Part as well?

      Is the work being done at a shop with a Guarantee? Is the Convertor reaching the light off point but the code appears anyways?

      The running lean code could be a clue as well.

      Someone really needs to look at the Data and figure it out, is really what needs to happen.

      Sorry I cant offer more


      1. The Cat is a Subaru part muffler isn’t. I have been now reading other posts and beginning to wonder. We had the headgasket done a month before the code appeared. They probably used non subaru gaskets and its possible they didnt do the whole tighten loosen step. They are an independent mechanic (Friend). Now I need to go talk to them to see what they did.

  52. Hi Justin,
    I have a 06 Legacy with 130,000 miles. In the last 10 days the CEL and cruise have come on and off twice. I had my regular Subaru mechanic check the code and it was 0420. He also checked the sensors and said they were ok. So he’s leaning towards the Cat being replaced. But just before the lights went on the first time I had filled the tank and added STP engine cleaner additive, which I had never used before. The lights were on until the tank was about half empty and then went off. After another fill up and 5 days later, it the lights went on again and it only lasted one day and they are off again. My mechanioc doesn’t think the additive would have been enough to make the difference in the fuel ratio. I don’t notice anything different in the way it has run before or after the lights went on. I drive about 60 miles round trip to work, mostly highway, and change the oil every 3k by the book. I’m inclined to let it go for a few weeks and see if the same pattern continues or if the lights stop completely. What do you think?


    1. Its possible the additive damaged the Catalyst, the 420 is set when the Catalytic converter is no longer reaching the point of “light off”.

      Its not a fuel ratio thing, not sure why that would come up?

      You could try using higher octane fuel and some spirited driving for a bit and see if that helps it.


      1. Thanks for your reply Justin. It’s been about 5 days now and I haven’t seen the light anymore. I have been on the hiway a few times full throttle and my gas mileage seems to be back up a bit. I’ll stay away from the additive I used and hope it’s back to normal now.

        1. Hi Justin,
          the lights came on again and I went ahead and had the new Cat installed.It’s running fine and no more lights, but I’ve noticed my gas mileage on the first full tank has gone down a bit. Is that normal with the new Cat? Should it get back to where it was once it’s broken in?


          1. Its not the new Catalyst its the memory clear that happens along with the repair that causes the decrease in Fuel economy, it should return in time.


  53. Hi Justin,

    Thanks for all of this information, it is really helpful. I have a 2005 Subaru Legacy with 112,000 miles and recently my check engine light came on. I took it to my local mechanic and said it was the 0420 code. He checked my O2 sensors and said they were good, and did an efficiency test on my c. converter and said everything was within the normal parameters. He wasn’t sure why the check engine light was coming on and suggested I contact subaru. I don’t want to replace the c.converter if it isn’t broken. I have already had the ECM flash done at the dealership. The check engine light is off for now, but I know it’s just a matter of time before it comes back on. Any suggestions?? Should I just bite the bullet and have it replaced? Many thanks in advance!

  54. Hi Justin,
    The CEL on our 2003 Forester is coming on again. It’s got 95,000 mi. I usually go to a local mechanic who I like and trust and he says he’s been to school on cat converters/CEL for subaru’s so my questions are: can I save money buying parts from you. Do you know of any independent subaru qualified mechanics in my area? Should I take it to the dealer or let my guy do it? He says it will run about $1200 and it’s best to ‘do all three’ at the same time. Tks, Jim Gordon Asheville, NC

    1. Hi Jim,

      I cant offer any savings on Convertor, there is no margin. It doesn’t have three convertors, it has a single piece primary and secondary convertor assembly.

      I dont know any one in NC, sorry.

      Dealer or good independent as long as Subaru parts are whats used.


  55. Hi Justin,

    Just found your website today, fabulous! I purchased a “new” used 2004 Outback 2.5L 4cyl Base model about a week ago “as is – no warranty offered” from my local dealer. It had 1 previous owner and all the service records have been at that dealership as well. 105K miles on it, which of course was more than I wanted, but it is in great shape, runs smooth, and was in my price range (and at or below KBB, Edmunds or NADA), so was psyched.

    ANYWAY, over the weekend I was driving thru town, when I had to abruptly stop to avoid someone backing out of a parking spot on the main street and my CEL came on immediately after this.
    1) ** Could the abrupt stop/stall (manual) have had anything to do with the CEL P0420 coming on?

    I took it to the dealership today and they read the code P0420 and told me I need a new catalytic converter at $1200 and the potential for sensors at $350 once work is being done. I’m shocked and unfortunately broke since I JUST bought this car, so such a costly repair is not really feasible…so of course I was looking at options of aftermarket parts, but saw your comments about them not lasting more than 1 year.
    2)**What is the life expectancy on the genuine Subaru part?

    Today they did not do the test for the emissions (only read the code) which I was unaware that reading the code and the test you mentioned were 2 different things of until reading your site. Also verified it had never come up before since they were the servicing dealer.

    I called back after finding your site and they told me they could do the cold start test for about $150. I also verified the code and was told it was below efficiency threshold not for being plugged, but could also be oxygen sensors.
    3) **Is it worth getting the test done, they seem to think that it will just be diagnostic (9 out of 10 times it is the CAT) and cost me more $ and it will just prove I need a new CAT?

    They re-set my CEL, so it is not on now.
    4) **Should I just continue to drive it, with higher octane (per manual recommended 87)until the light comes on again? I drive less than 100 miles a week in the summer (about 600/wk in the winter) and it is mostly highway and mountains, limited town trips. They said could come on in 2 days or 2 months, or longer. Unfortunately, I got the feeling from the dealership that I could really screw up the car if I don’t replace it. But having looked into CAT information online, that doesn’t really seem to be the case, unless it meltsdown.

    5)**What is the likelihood of an actual meltdown? And if that happened what kind of $ am I then looking at?

    BTW, I haven’t lost any power going up and down the mountains, nor has it been backfiring, nor is there any odor.

    Thanks in advance for your response.

  56. Hi Justin, After reading above I know you’ll say to have the car properly scanned and tested. The problem here is I’ll have to go to the dealer because I can’t find a proper shop to do this.

    When I bought this car from the original owner, my sister (2002 Legacy wagon with 150,000 miles) she just had another cat replacment a her local shop. I though great! it’s taken care of and should last a long time. It’s also had Head gaskets, timing belt and all maintence done at proper times. I have her note book of every repair and maintenence item recorded with all the receipts. It Runs great and doesn’t use any fluids at all.

    Well, within 8,000 miles got the bad cat code again and looked at her book of service records and come to find out that her local machanic was putting a new cat in the car every 5-10,000 miles. These are obviously aftermarket and he only charged her for the 1st one, about $600. The car is now on it’s 5th cat. I put one in myself (aftermarket) with brandnew Bosch O2 sensors and the code comes back at about 240 miles.

    Anybody that has pulled the code just wants to replace the cat or sensors with theirs. I’m really frustrated with this and It’s got to be something else. One guy’s scan tool could see the sensors switching so he said they are working just replace the cat.

    Any suggestions you have would be greatly appreciated. I don’t want to put over $1,000 into a $2,500 car. All I want to do is get through the state of CT emissions test!


    1. Tom,

      Aftermarket Catalytic convertors are worth $ZERO.

      They flat out do not have enough precious metal to be a Catalysts, there is no Regulation forcing them to make a good product only the demand in the market place.

      Buy an O.E. Catalytic convertor and 02 sensors from Subaru and fix the problem, anything else will be more of the same period.

      I understand trying to save money, but there is no solution other than back to factory parts.


  57. The check-engine light on my ’05 Outback came on two days ago. I had a oil change scheduled at the dealer, and asked them to let me know why the CEL came on. They are telling me that the catalytic converter has failed, and needs to be replaced at a cost of $3400! Later that day, the light no longer appeared. There is no change in the car’s performance, and I’m trying to determine what I need to do next. Any suggestions would be greatly appreciated.

    1. Hello Debbie,

      $3400 sounds excessive unless its a H6, or rust is an issue?

      If its not affecting the performance, you can put it off until you need to test for emissions if you live in an area that tests. In the mean time I would look at buying this so you can periodically scan it your self for codes to make sure nothing has changed. The light may go on and off until its resolved. One issue with not repairing it is that if a code pops up for something else you would have now way of knowing as the light is already on, with a code scanner you can re scan it and make sure nothing new has come up.

      Hope that helps


      1. Hi Justin,
        My outback is a 2.5i, not an H6, & rust is not an issue. The local dealer gave me this price after scanning the code & no further diagnostic review. Not pleased with them. There is no change in the performance. Someone else suggested it could just be the oxygen sensors and not both catalytic converters, as the dealer suggested. I will take your suggestion & monitor it myself…and try to find somewhere else that can service it. I appreciate your reply. Thank you

          1. hey justi, we have a 2007 legacy with 180000, we were driving home and the car felt like it was holding back and wouldnt get up past 40mph and made growl type noise at passenger side airbox, the engine sounds perfect just no power dealership replace mas sensor and tol me it was fixed went to drive home didnt make it out of lot, whats your opinion?

          2. That the Dealer should have another look.

            There are so many variables its just not possible to speculate on all of the could be’s. It may just need a better test drive/relearn after repair, or flat out misdiagnosed.


  58. Hi Justin,
    In late April I took my 2007 Forester, 59,000 miles, to a Subaru dealership to address recall ‘WVU-31’ from April 2011 (yeah, it took me a while!). They reprogrammed the ECM.

    One month later the Check Engine light comes on–that’s never happened before. Being Memorial Day the only thing I could do myself was tighten the gas cap. After a couple of short trips, the light went off (along with the blinking cruise). Thought myself a genius!

    Except, two weeks later, it’s back on. Happened Sunday as I rounded a curve. Tightened cap again–light went off after three short trips. But it’s back on, triggered by, seemingly, making a simple right turn.
    Baffled! What gives? Thanks!

    1. Hi Larry,

      If you have read this article or any of the other ones I have written about the check engine light you will see that I suggest a code scan in correlation of the light coming on.

      Tightening the gas cap if its loose may correct the light , but more importantly why would you keep leaving the cap loose?

      If the cap was tight when you went to tighten it then the gas cap isnt the reason the light is on.

      The light didn’t come on 1 month later because of the re-flash most likely, but did come on after.

      You wont find any answers until the code is read and a plan made form there.

      Here is a link to a basic code scanner you should consider purchasing if you want to be involved in some of the aspects of your car needs, any one with a 1996 and newer vehicle should own one.

      Hope that helps


      1. Thanks, Justin. Yep, need to get that code read!

        I am always vigilant about tightening the gas cap for ‘several clicks’. Always. And yet the first couple times I checked it when all this started happening, it DID seem loose!
        Could, over time, a cap’s plastic threading weaken/degrade allowing it to work itself loose with regular driving?!

        BTW, there was no link in your reply.

  59. Hello Justin,

    I’m running 02 Impreza 2.5TS, with EJ251 for about 6 months now. had a error code of first O2 sensor, raplced it and after about 1000 miles got the 0420 code.
    did some reaserch and fell lost for ideas.
    my CAT is good, I’ve changed the secondary O2 sensor as well, searched for leaks before O2 sensors, did not find any. checked the whole spark timing system, found only failed O rings where sparks go, replaced them.
    checked the injectors and fuel presure they are fine.

    after all of this the light did come back today. its always on country lanes or motorway, never in the city, always on light gas…im out of ideas, please could you through some of them?

    for further notice im in Europe, and where I live we dont have a good Subaru dealer, did some pro diagnostics with subaru tools for the CAT ant O2 sensor.
    I work in a garage and been driving subarus for couple of years know.


      1. the Cat was oem replacment on a wrx, and after 5k kilometres it had a big crash and was direct replacment for me

        1. Hi Oggy,

          New or recent doesn’t always mean its ok, it needs to be tested to know if its good. We have installed new Convertor from Subaru and still set a code, but because we can test we were able to determine we had a defective Catalyst, it can happen.


          1. What I am trying to explain to you is that just because a part was replaced in the past does not mean that its ok today. You cant know if the Cat is good or bad without testing it.


  60. Howdy Justin,Thanks in advance for reading and advice.
    I am owner of 2001 forester w/180k mi.(just purchased recently).currently uses no measurable amount of coolant or oil.It seems to run fantastic!Car was owned by a easy driver and kept in top condition. shortly after I purchased c.e.l. went on…code po420. and I am hoping for easy fix.previous owner says c.e.l. was not on b4 now.
    I have a few Questions to help educate myself of the safest rout to go.

    Are there any symptoms to watch for h.g. failure , my forester has never had replacement can I make head gaskets last?
    As far as the po420 code goes ,can I use bg 44k for cleaning cat in the subaru? I have had good results with previous auto injector carbon problems, is there a better product for cat cleaning to use along with running it hard? might it cause premature head gasket failure?
    Also,should I be running hi octane fuel full time?

    Thanks Alan

    1. Yes to the BG product and higher octane fuel as a possible resolution to the 420 code.

      Keep the fluids in good shape and that is the # 1 thing you can do to prolong the head gasket life.


  61. Hello,

    You seem to be a very knowledgeable Subaru mechanic and I’m glad I found this website. I have a 2003 Subaru Forester with a P0420 trouble code. It has a HG leak, I lose coolant, not at a very high rate though. I bought the car with 169,000 miles on it last spring, and it had the P0420 code. The mechanic told me I needed a new catalytic converter, because a friend suggested I try using a spark plug non fouler on my rear O2 sensor to clear the code… Didn’t work. So I got the cat replaced and the code went away…

    The code is back now, P0420 again. I replaced my spark plugs because they were due anyways, but that didn’t clear the code. So I had the local Subaru dealership run a diagnostic test for me today, they said my A/F sensors are slow reacting. (Those are the O2 sensors, right?) And the HG’s are leaking. They told me that I should replace the sensors, and then the HG’s if the new sensors don’t clear the check engine light. He said that the HG leak could cause the Catalytic Converter to fail. Do you think I’m looking at a new Converter as well? It’s still under one year since I replaced the cat. Should I have the cat replaced since it’s still in warranty? Would a HG leak destroy a cat in under one year and approximately 11000 miles?

    What do you think I should start with? The O2 sensors, or just replace the HG’s and see if the code clears? I appreciate your input.

    1. Hello Spencer

      You didn’t mention if the Catalyst was replaced with a OE Subaru part or a aftermarket Catalyst?

      If you used a cheaper Aftermarket Catalytic convertor than yes its most likely already failed, we generally see the aftermarket Catalytic convertors last a year or so if its under warranty than yes have it replaced if it fails the tests I have outlined in the Article.

      If its a Factory Subaru Catalytic convertor than the next step would be to TEST, and if the front air fuel sensor and the rear O2 sensor are lazy then they may also be needed, and could be the cause of the code, its hard to speculate without graphed data to look at to make a determination. I would just be guessing.

      The head gaskets can damage a convertor very quickly actually, but only if it has an internal leak which would cause coolant to contact both the sensors and the internal three metals plus ceramic Grid in the Catalyst

      Hope that helps


      1. Hello Justin,

        I had the converter done at just a regular shop in my area. Not a Subaru dealer, so I think it was aftermarket. I didn’t know the converters were made to only last that long. Is the one year lifespan of the aftermarket converters common, or just when there’s a cause like a head gasket leak to make them fail?

        And could a head gasket leak damage my sensors? I need to know what I should do first. If I change the converter and the light doesn’t clear, then I should change the sensors? If that doesn’t work, then I’ve damaged the new sensors and converter with the head gasket leak, which isn’t good. Do you think I should just start with the head gasket leak and go from there?

        I apologize for all the questions, especially without having all the proper information to give you to make a definite decision. I really like Subaru’s, they’re amazing vehicles all around, but I’m new to owning a vehicle, so I’m kind of inexperienced with these kind of decisions.


        1. Hi Spencer,

          The aftermarket Convertors just dont have enough of the three precious metals to work as well and as long as the Subaru OE. One year life span for the aftermarket convertors is very typical and very unfortunate which is why we wont use them.

          The head gasket leak on your Subaru needs to be repaired, most likely the convertor and sensors as well. The failed convertor wont do anything to leave you stranded, or do damage by waiting to repair it other than the air quality which is why its there in the first place.

          I would make the head gasket repairs, then budget for the Subaru convertor when you can and maybe replace the front air fuel sensor and rear oxygen sensor at that time as well. If all is done well with the RIGHT parts you should be looking at this in the rear view mirror for years to come.

          Hope that helps and best of luck going forward.

          Your Independent Subaru Expert

    2. You should be aware of a Subaru recall on head gaskets: recall # wwp99. If your car complies, Subaru will pay part or all of the cost of a head gasket repair. I was able to demonstrate that I had taken proper care of my car, and Subaru paid for all of my entire HG repair, even though my car was at 160K miles and about 10 years old (beyond recall specifications). I had to demonstrate that they should have advised me of the recall during a timing belt replacement in 2008. Good luck!

  62. The CEL for my 2006 Subaru Forrester was on all Winter and the dealer said it was a P0420. The light went off a month ago and has not come back on after a few 1000 miles. Will this pass inspection or is the history still in the memory?

  63. Hi Justin,

    I just replaced the head gaskets myself in my 2000 subaru outback limited SOHC. Now after driving it for two days I have a p0420 code. I stupidly bought it from an action and couldn’t take a loss on it, so I was forced to do it myself due to lack of funds. I bought a fel-pro head gakest kit and changed all the gaskets on the way in as well. I pretty much went through the whole engine timing belt, water pump, cluth, ect. There is no more external oil or coolant leaks at the head anymore. But the code has me puzzled. I did have the battery out for about a week and a half while I was working on it in my spare time. If it is because I had the battery out for so long what do I do to fix it. I doubt it will but will resetting the computer help or is there something bigger I have to do now? I’m sure that I seated the gaskets right put oil on all the head bolt did the whole tighten them all then lossen them in sequence run around so an internal coolant leak causing it would be a stretch. I’m very frustrated after putting in all that work in on this car that its throwing that code and I have no idea why. So any advice would be much appreciated. If you don’t have any good advice I have a 2000 subaru outback for sale. I’ll even give you a good deal…

    Thank You for your time.

    1. I wish you would have purchased the gaskets from us, the brand you mentioned is just not a good option for Subaru and may in fact not be sealing.

      You can try to build up some exhaust heat with a spirited drive in lower gears and higher RPMS then typical and see how that goes.


  64. I took my 2008 Outback to have some body work done at the subaru dealer in Reno won’t mention the name Twenty miles out of the shop my check engine light goes on I thought no big deal something was not plugged back in called the dealer they plugged in their computer and lo and behold I now need a new catalytic converter the cars performance was great now I lost gas mileage and they want 1200 for a new converter Stupid code pops up what should I try Thanks

    1. Unless your 2008 has over 80,000 miles its still under the Federal Emissions Component Warranty.

      If its over 80,000 miles than you should budget for a Catalytic converter Replacement in the Future.

      You can try higher octane fuel and harder driving to build up Exhaust Temperature.

      Hope that Helps


      Your Independent Subaru Expert

  65. how can i repair this problem in my big car (check Engine ECU at next stop)Model:VOLVO 2004, Size:420,that chech wrote on screen at my car but i can not know about this problem,,,, pleas answer me quichly, Regards…

  66. You mentioned further testing is not free.
    What should I expect to pay.
    I like to know before I decide whether
    to pump money into my 2005 Outback with 190,000
    miles on it.
    It my be time for a new car.

    1. Hi Jim,

      I cant comment on what you should expect to pay other than at my shop, we charge $96.00 to perform a Catalyst function test in conjunction with graphing data from the 02 sensors.

      Most times if we make the repairs the same visit we will discount or waive the diagnostic fee. If all we do is Diagnose it we still charge the $96.00, we just try to be fair.

      Now, I own and run a good shop that cares about our customers, If you go to a Dealer it will be higher, if you go to a general repair shop it may be higher still and not correct, if you find a version of my shop near you, than you should expect to pay close to what we charge if not it will be different.

      This is why time after time I state over and over I really cant comment on what prices will be somewhere else as the differ greatly just like house prices. You really just need to research this out in your part of town.

      I hope that helps


  67. Hi Justin
    Great to find this information here. I am in New Zealand, and have a (Dec) 2006 Tribeca 3.0 liter 110,000 km (68,000 miles) and two weeks ago had the CEL come on. Diagnosis by the local dealer P0420.

    Quote to replace both converters – a staggering NZ $9000 (US$7500.00).

    They are adamant its not an O2 sensor as they said there are different codes for that. They said they have NEVER come across one failing this early. They are a large and reputable dealership – Anyway they cleared the codes and told me to monitor it. 400km’s of mainly open road driving and the CEL light is back on again. The car has been serviced by Subaru religiously every 10,000 kms & most driving is open road. High octane fuel has always been used. Performance and fuel economy are normal. We do not have an extended emmisons system warrenty here as it seems you do in the USA.

    So I am hoping you can suggest what my next moves with my Subaru dealer here should be as i am very reluctant to rush into replacing the CC,s. What other testing should they do? Also do Tribeca CC’s cost that much in the USA i.e. it could be worth importing them myself – from what I’ve read online most Subaru ones appear to only be around the $1000 each mark.

    Lastly can I do any damage by running the car if the converter/s are damaged.

    I would very much appreciate your help here.

    Thanks very much


    1. Hello Craig,

      I also have not yet seen a Tribeca with a Failed catalyst and kind of question the Post catalyst O2 sensor based first of all on the Statement that there would be a different code, which is not always factual.

      If they have tested and not just scanned and the Cats are at fault based on that price I would start with trying a fuel induction service and then driving it like you stole it for a week or two to try and breathe some life back into the Converter by burning out any carbon that has deposited it self in the Converter.

      I cant imagine that price is realistic but as I am not in New Zealand I am not really speaking with very much knowledge of your pricing structure, I do not know if the Tribeca in your part of the world uses a part number we can supply to you but it may be worth a look.

      Hope that helps


      1. Thanks for your reply Justin.

        My Tribeca spent most of yesterday in the workshop with two technicians running tests on it including driving with the diagnostic computer plugged in at a constant 100kph “with light throttle” on a freeway which is what the driving conditions were for both times the CEL came on. They also swapped O2 sensors from LHS to RHS and still got the same reading. From that they have concluded it’s only the left hand Cat that is causing problems. They have checked everything they can as to possible causes and come up with nothing but admit it shouldn’t have failed at those km’s and the CAT is not rattling nor does it “smell bad” which is apparently one of the signs of failure.

        They also tried to source ECU remap codes that would just “ignore” the reading from the O2 sensors which is how they have solved issues with other models (our emissions laws are not quite as strict as USA or Europe) but because the Tribeca is built in the USA not Japan there is nothing available. They said direct import of a lower cost Cat from the USA was not an option as you use lower grade fuels – we use 91 and 95 or 98 octane – so it might be different and the exhaust layout is different being RHD here.

        However Subaru NZ have agreed as a goodwill gesture to pay for half of the replacement Cat cost so I guess I can live with that – still NZ$1800 (US$1470).

        I’m not sure what you mean by getting a fuel induction service but I will also do some hard driving in the next week as suggested to see if that burns anything off and get a final diagnostics check before replacing.

        Thanks again


        1. Hi Craig,

          I have just had this code come up on my 2007 Tribeca.
          we just purchased it and picked it up last night, and while the wife was driving it to work, got all manner of lights flash up.
          A diagnostic check gave us the (what seems like standard with these cars!) code of P0420.
          How did you get on with your problem? Did the techs come up with a solid fault?


  68. Hello Justin,

    I came across your article while researching the issue I am having with my wife’s 03 Forrester. I love the car, but the head gaskets were leaking. Last week I finally had the time and money to have my friend who is a former head mechanic at ford (and also owns and works on subarus) change the head gaskets, exhaust gaskets, timing belt, water pump, etc. Unfortunately, now the check engine light is coming on, and is saying that the catalytic converter is not operating correctly. above you mentioned –

    “The next part is a little less scientific and a little more in the grey area, its entirely possible for a check engine light to occur after the battery is disconnected and set a PO420 even though there were no indications of a catalyst issue prior. The why is learned data that has been lost typically with light of parameters, coolant temperature and maybe engine repairs that make changes in Combustion temperature such as replacing leaking head gaskets. But I will save that for another article.”

    Any chance you have that other article around? After dumping money into these repairs, last thing I want to find is that I now need more expensive repairs.

    I have filled the tank up with 93, and added a bottle of chevron techron.

    I may drive the car around a bit at a higher rpm, as was mentioned above.

    Any thoughts you have would be a huge help.


    -Matt P

    1. Hi Matt,

      I have not posted another article about OBD II, they just are to technically difficult for the average non car person to get their heads around and overall not well recieved. Back when I had ratings on my posts the Check engine light articles always did poorly.

      I wish you would have tried us for the parts for your repair.

      Best of luck


  69. This is the only place on the web where I found a reference to my issue! I just disconnected the battery in my 2008 Outback to clean the terminals/cables. After reconnection, the CEL came on with P0420 code. I took the cables off to clean them again and a day later, same thing. Had the code cleared and now 2 days later the light came back on. Any more info on why you mentioned this could happen? Even if it’s not scientific, I’d like to know. My car is still within the Cat warranty at 76K, but I have a rebuilt title and I’m not sure they will honor it. Thanks!

    1. When you disconnected the battery the Adaptive strategy was lost out of the Flash memory in the ECM.

      On a modern vehicle you are asking for trouble if you don’t use a “memory maintainer” tool when disconnecting the battery.

      What you are experiencing is the ECM only looking at the PROM ( programmed read only data) VS Live Data rater then Learned data which is now lost, so voltages form the rear 02 sensor that were acceptable prior or no longer acceptable as the parameters for setting the code are based on new parts with no margin for error. This would be that based on this condition X, Y shall be this voltage. Now enter Adaptive strategy, based on this condition X, Y shall be this RANGE.

      After driving the vehicle hard for a bit it may go away, but you most likely had a “”on its way out Catalyst” and now that the ECM is merely looking at “pass fail” data in a narrow way its easy to spot the issue. There may also be some firm ware update for your car as well that may address the issue.

      Hope that helps


  70. Great site Justin..and wish you were in Va.
    Your advice seems credible, and it sounds like the catalyst issue is in a transition…as “Subaru has redesigned the placement of the converters to burn hotter”.
    I’ve wondered if the newer ethynol mixtures contribute to the problem, and if so… what the car makers are doing to correct the issue.
    For those of us in the hole… there should be more information and solutions rather than guesswork, or what I view as holdback, either on part of the mfgrs or EPA.

    1. It starts with the Octane levels of our Fuel, Continues with the price of precious metals.

      Like I keep stating, if instead of the price on the sticker when you buy the car instead stated how much to own over 10 years no one would buy it, this goes double for German cars.

      Clean air costs money the media doesn’t discuss it, so most are unaware. The regulations for new cars also increase every few years as well which make it very difficult as what was perfected two years ago must now be redesigned to meet tougher standards.



  71. Hi, Justin, thank you for info you have provided on this site. Can you tell me what is the drive pattern to complete the Catalyst Monitoring? Thank you.

  72. Suffering the same problem with the P0420 code. My 08 Outback has 90,000 miles, so conveniently out of warranty. Dealer checked and said that yes, it might need a new converter. CEL has been off for about 2 weeks but came back on last night (no driveability problems). Of all the vehicle I’ve had over the years with much higher mileage, I’ve never had to replace a converter. Based on all of the hits on the web regarding this Subaru problem Iwonder if there’s a secret warranty out there?.
    Thinks this is a great vehcile, but is it reasonable for the converter to fail after this kind of mileage?

    1. Hi David,

      We save as many convertors as we replace. Other makes are experiencing the exact same issue Subaru Is not alone in this and its he price of clean air. The only way to cut down on this is to put more precious metal in the Catalyst which would greatly raise the price. Subaru just like Toyota, Chevy, Ford etc uses the minimum amount of platinum, palladium and Rhodium to get the Job done, Google or Bing any car maker with the term P0420 after it, you will see what I mean.

      A car you owned in the past had less technology and less strict emissions requirements.


  73. Thanks for your very informative article. If you ever get around to that “another article” about PO420 coming on after an ECU reset, I’d be delighted. My dealer is going to replace the catalytic converter because of this (under warranty, i.e. at Subaru’s expense) but I’m not so sure that it’s really necessary.

  74. Hi. I have a 99 outback with 144k miles. The cel appeared a few days ago. The local Auto zone reader reports a 0420 code. I had the c converter replaced about 2 years ago. What I also noticed is now is that my temperature gauge is not working. Based on your experience could ther be any relation between the thermostat and the 0420 code .


    1. The thermostat being stuck open could affect the catalyst function but not the temperature gauge function, it could cause the gauge to read a lower temperature than normal but not cease to function.

      Most likely you have two issues?


  75. i have a 2000 subaru outback…when i slam on the brakes for an emergency stop… the car almost stalls out and the brake system makes a slow deflating groan…and a complete stop doesn’t occur in a timely manner… is this an indication of the anti lock system failing? brakes work ok when applied normally. the car has 121K on it. been maintained.

    1. Really cant speculate on whats the matter, its possible the booster is failing but that would be very uncommon.

      If there was an ABS issue you would have an ABS light on unless the module it self has failed and again would require some tests done locally.


  76. Dealer reports teh Engine light went on today due to P0420 code. Testing now. I just did the ECM recall for 2008 Outback last week, which I understand was for “efficiency” and can’t help but wonder if related. Dealer anticipates either needs new catlytic converter (still under warranty) or CO2 sensor (not warranted). Any advice as I go chat with them — and likely reach for my wallet?

    1. Typically the Catalyst and 02 sensors should be done a the same time. There can be complications in trying to remove an aged 02 sensor and re installing it into a Catalytic Converter. The warranty complicates things and can create a situation where the Dealer will want to try the 02 sensors and it may temporarily resolve the issue and get the vehicle out of the warranty period.

      The premise behind the logic update was to stretch out some of the parameters for when the catalyst function is monitored as in some cases its not truly being tested properly.


      1. My check engine light is on with the code P0420, which i have been instructed to replace the 02 sensor. My question is: is it normal for the Cruise control light to be on and blinking (not functional) and the Vehicle Dynamics control (vdc)light is on and non-functional? What do you think of this??

        1. Hello Stephanie,

          Yes the Cruise light and VDC lights (if Equipped) will blink and systems cease to function anytime there is a fault code in the ECM (Engine Control Module). This is to get your attention as the public tends to ignore the check engine light or think they just have a loose gas cap, then the next day they are stranded.

          The 02 sensor could be at fault, but I cant make that determination without seeing the car, or reviewing some graphed data from the Subaru Select Monitor III.

          I would also suggest reading in your Owners Manual What Subaru states about not just the Check engine light, but about all warning indicators so you have the knowledge going forward, and know when its ok to drive and when its not ok to drive.

          Hope that helps


  77. Justin.
    I really like your website and it has provided a lot of information to me on the P0420 code on my 2000 Subaru Outback. I would very much value your input on what I should do with respect to the recommendations I am receiving.

    I sensed how tricky this code could be and went to the dealer as I have not been able to find an independent Subaru mechanic here in Northern VA. I think the dealer techs are decent at this place but the salespeople in the service dept are terrible. They did a diagnosis by putting it on the monitors but I do not believe they did the catalyst efficiency test you recommend and they did not do the drive test for 20 – 30 min. I did manage to speak directly to the technician and he seemed knowledgeable and thought that the front and rear sensors were ok, but that the catalyst needed to be replaced. He said it often is the sensors or an exhaust leak but these seemed to be operating fine and there was no exhaust leak evident.

    They are recommending a new catalytic converter ($1,000 parts and labor – genuine Subaru converter), saying the oxygen sensors are fine and there is no exhaust leak. They are also recommending a reflash of the ECU ($95).

    The car drives very well, there is no smell from the converter. I have heard that sometimes reflashing the ECU can take care of this P0420 code reflash problem so I am thinking of trying that first.

    Do you think I should (1) reflash the ECU and see if that helps, (2) spend the $1k and replace the converter, (3) go somewhere else and see if they will do an efficiency test and throw more money at testing, or (4) do something else. The dealer said they do not do the efficiency test. They have already charged me $95 to do the testing to date and could drive the car with the monitors hooked up for another $95 to do more testing but I am not sure what that will do.

    Any thoughts you have would be most appreciated.



    1. Hi David,

      The place to start is the Reflash. Unless the Catalyst is getting no where near the light off point? We would suggest the logic update first as its a lot of money. If after the reflash it didnt resolve the issue I would then re evaluate it, and if the Catalyst is still not reaching 600 degrees I would then replace the Catalyst.

      Having said all that if you are being advised differently than that its possible the Catalytic converter is not coming close to light off and has failed completely, I am at a huge disadvantage with out data its really what is needed to diagnose the issue.


      1. Justin,

        Thanks again for your response. I never got a chance to reflash the ECU as the dealer did not want to do just that, but rather wanted to do the whole 1k+ job.

        I drove the car in second/third gear on a local 14 mile roundtrip Monday night. Tuesday, I took the car on a long trip, over 300 miles and after restarting the car 100 miles into the the trip, the light was off and did not come on. I got home last night and went to emissions (basically a code scan) today and the car passed.

        My wife drives the car now only for short, local trips and not much highway. Is it possible that the higher operating temperatures from driving the car at high rpms and being driven at highway speed, somewhat cleaned out the cats and brought them back within the specifications of the ECU?

        The car also had leaking head gaskets and we have always changed the oil frequently (2.5k – 3k, never as much as 4k miles). I am wondering if these contributed to the deterioration of the cats. I have read that the leaking head gaskets and the additives they used to put in the oils cause the inside of the cats to deteriorate. I thought I was being proactive in changing the oil frequently…

        The 2000 outback has 107k and is running very well so I hope to keep it for a long time.

        Thanks again,


  78. Hello, Justin, thank you for some terrific info you have provided on this site. I only wish you were in New England instead of Seattle.

    Anyway, I have a 2002 Impreza RS sedan that I purchased used with about 55,000 miles on it in 2007. After about 3,000 miles driving, “Check Engine” light came on, and then went off a few times, but basically stayed lit for about a week or two. P0420 code was detected, and Subaru dealer eventually performed catalytic converter replacement under warranty.

    At 60,000 mile maintenance, dealer replaced valve cover gaskets and timing belt. At 75,000 mile maintenance, Dealer replaced head gasket under warranty. Last fall, the car went through 90,000 mile maintenance OK at the dealer.

    Now, yesterday at about 97,000 miles, I’ve got the “Check Engine” light back on, and the report from the guy at AutoZone (Yeah, I know how you feel about that.) reads:

    P0420 Catalyst Information Below Threshold
    Probable causes
    Rich A/F Ratio
    Ignition System malfunction
    Faulty Catalytic Converter

    Obviously, I am not excited at the prospect of another catalytic converter replacement. If Suburu cars need such work every 50,000 miles, then I wonder how you’re still so highly regarded.

    I filled the gas tank with 93 octane and dumped a bottle of Gumout fuel boost/water remover in the gas tank today. The car is running fine, no problems. Should I switch to 89 octane? Should I take it out for a long drive? Anything else I can do before I bring it back to the dealer?

    BTW, in terms of usage, I commute 60 miles round trip daily, mostly highway, and otherwise drive around town a bit. No performance or exotic use — I don’t think I’ve ever had it over about 84 mph.


    1. Hello Phil,

      First to your statement below

      “Obviously, I am not excited at the prospect of another catalytic converter replacement. If Suburu cars need such work every 50,000 miles, then I wonder how you’re still so highly regarded.”

      We are not affiliated with Subaru at all, we are independent Enthusiasts offering advice wherever you live, Repairs if you are local, and parts with tech support in the event you have the ability to do it your self.

      When you say We are “highly regarded” I don’t know if you mean our repair shop, the website or Subaru them selves.

      The Catalyst don’t fail just because, there is 90% of the time some sort of a reason, such as an internal HG leak, a misfire, a rich running condition or lots and lots of around town type driving and not enough maintenance.

      I would say yes its uncommon to need a second Catalyst at 50k after the first but I don’t know if the front air fuel sensor was ever replaced or if there is something else going on? Also the HG failure could have degraded the Catalyst as well, but the most common reason they fail is they get all choked up with carbon and crud and no longer reach the light off point. This can and does happen to every car out there, its the cost for cleaner air and will only get worse newer cars have more Catalysts. Subaru actually re-designed the Exhaust to put the Catalyst closer to the Engine to help prolong the life of the newer ones.

      I would start with the higher octane fuel, some driving at higher RPMs at lower speeds and see if you can breath a little more life out of your Catalyst. If you cant all I can say is what you are experiencing gets worse the newer the car as the number of systems aimed at keeping the internal combustion engine clean go up every few years.

      Subaru makes a Great Safe & Reliable AWD vehicle. I cant think of any other I would rather be in if I was ever in a Head on accident or out in the snow. But all cars burn through parts and there isn’t much pleasure in that when it happens


      1. Justin,

        Sorry for the confusion regarding Subaru vs. your independent shop. That’s what happens when I post after my bedtime!

        You make an interesting point, “it’s the cost for cleaner air and will only get worse newer cars have more Catalysts.” This is certainly going to affect my thinking when it comes to possibly purchasing a used car in the future. Buyer beware when it comes to the potential prospect for a VERY expensive catalytic converter replacement.

        On to the good news. Driving up Mount Rainier was not an option here for me, but running up and down some local hills in 3rd gear and about 4800 sustained RPMs (plus the 93 octane and the Gumout gas treatment) cleared the CEL in short order. It’s a week later, and all is still good!

        Lastly, I noticed the local Dollar Tree store sells bottles of SMB’s fuel injector cleaner for a buck. Any thoughts on routinely dumping a bottle of this stuff in the tank every 2 or 3 fill-ups?

        Thanks for your help, and your great site.

    2. My 06 outback kept tripping the 420 code off and on for a year. Had a local garage check it out and they could not find any issues.
      I just changed the oil and decided to replace the PCV valve while I was in there. They CEL is now off and has been for about 4500 miles. For $6.00 it’s worth a try..

    1. Have a 420 code.Mech says to replace cat.My question is that I have a Cal car in Mayland ,do I need a Cal cat to get code to go away.Thanks

    2. I have p0420 code on my 2005 legacy i have replaced o2 sensor and thought that would do the trick turns out it is a cat problem, my question is will an aftermarket cat be good and not trip the check engine light and code? i’d really like to stay away from the OEM $400+ part if i could do it for less by just replacing with aftermarket.

    3. My 2004 Legacy keeps showing a P0420 code. Here is the sequence of events:
      – I had a misfire and noticed the CAT downstream sensor glowing red when I was able to pull over. P0420 was showing
      – I replaced plugs, wires, and eventually the ignition coil
      – everything was fine for about 200 miles
      – P0420 returned, and I tried “CatClean” in the gas tank
      – again things were fine (coincidentally? for around 200 miles, and then the code reappeared
      – replaced the upstream air/fuel sensor, and the same thing happened

      The car continues to run great. With 230,000 miles on it,I can’t complain. I see comments about an ECM reflash. Is that the next step prior to investing in a new CAT? With age and mileage, I’m trying to fix this cheaply.

      1. Hi Steve,

        If the cat is tested and fails the test than it needs to be replaced, if how ever it reaches light off and still sets the code maybe it’s a candidate for the re programming as a first step. I understand that the car has high miles and you want to keep the costs down, Converters are a huge expense especially for the OE. Another thought is to not fix it?


Leave a Reply

Your email address will not be published. Required fields are marked *