2004 WRX P0420 have replaced everything and won't go away
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This is a discussion on 2004 WRX P0420 have replaced everything and won't go away within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; So, I got the dreaded p0420 on my 04 WRX Wagon that I got about 9 months ago. I am ...

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    2004 WRX P0420 have replaced everything and won't go away

    So,

    I got the dreaded p0420 on my 04 WRX Wagon that I got about 9 months ago. I am pretty confident the dealer that sold it to me cleared it so it wasn't noticeable if you didn't know to look for it when you test drove it. Came on about 200 miles after I bought the car. Didn't realize the nightmare it was or I would have gone striaght back to them. 6 months later still fighting it I realize I really botched it waiting to long to put it on them.

    So, first attempt was replace the rear 02 sensor. Cleared the code, came back 200 miles later (basically one cycle I think).

    Second Step was new rear Cat, got a Magnaflow made to exact OEM specs according to them and it was the exact same size and whatnot. Cleared the code, came back in one drive cycle.

    Step Three, replaced the front 02 or fuel air sensor depending on who you ask. Cleared the code, came back in basically one drive cycle.

    Step four, replaced the front Cat, Dealer said this was the problem, so did a cat shop who checked the temperature. Cleared the code. Came back after several drive cycles or about 900 miles of driving. The cat shop says that there is a precat before the turbo? Anyone know anything about this. The car had 164k on it when I got it. It has 170k on it now. There is no indication of any modifications what so ever, one owner car, mostly dealer serviced. Had Subaru cats on it when I got it. Not even the air filter has been modified so I seriously doubt any tune or exhaust mod has ever been done. Replacing the front cat actually did improve the mileage which makes me think this was definitely bad and contributing or would throw it. THe car runs well, though occasionally I get a slightly rough idle. I have put a temp gun on the manifold and all four cylinders are running about the same temp so I don't think there is a cylinder not firing or anything causing that, it could really be fine there and I am just overly sensitive as I am frustrated trying to figure this out. Not sure who to turn too, the Dealer just pulled the code and said put a new one of these on. No actually diagnosis like checking the cat with a temp gun. Thoughts?

    Oh, it has a new timing belt and water pump, new ac/belts, etc. I haven't changed the plugs yet, that was gonna be next.

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    Registered User Obeisance's Avatar
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    You checked for a lack of firing by using a temperature gun. When the car is warmed up, shouldn't the coolant evenly distribute the temperature throughout the engine block?

    There's another sensor that you didn't mention replacing: the pre-turbo temperature sensor which may be used to monitor the pre-turbo catalytic converter health.

    Also, the general consensus is that the cat before the turbo is a liability (clogging or breaking apart and roughing up the turbo), thus it should be removed (and a 2.2 kOhm resistor placed where the temp sensor plugs into the electrical system). Search for uppipe replacement and you'll find lots of threads.
    David - '05 WRX

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    Thanks on the pre-cat. Well shouldn't the exhaust manifold be cooler over a cylinder not firing as there is no hot exhaust gas going thru it?

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    Registered User pfsquirrel's Avatar
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    Any resolution on this?

    I've been getting this as well on my '02 bugeye. Because of this I'm also getting the lean mix code.

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    Quote Originally Posted by pfsquirrel View Post
    Any resolution on this?

    I've been getting this as well on my '02 bugeye. Because of this I'm also getting the lean mix code.
    I haven't had time to deal with it yet. been slammed. My plan is to figure out the resistor part of the uppipe fix. Clear the code. Give it 1k in miles and see if that does it. If it does, order an uppipe and have it installed. Not sure if that is the best plan, but dang if I want to do the whole uppipe and it still come back.....

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    Moderator Donkey's Avatar
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    Your wasting your money throwing parts it. The P0420 is for the rear O2 sensor only. If the car is completely stock I would have spent $15 on a mechanical CEL delete or this electrical one and called it a day.

    http://www.bigdaddiesgarage.com/straight-cel-fix.html
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    Quote Originally Posted by Donkey View Post
    Your wasting your money throwing parts it. The P0420 is for the rear O2 sensor only. If the car is completely stock I would have spent $15 on a mechanical CEL delete or this electrical one and called it a day.

    http://www.bigdaddiesgarage.com/straight-cel-fix.html
    MIL-Eliminators.com
    Help me understand how this is not just hiding the problem or making the code go away. There is still a part, and the one now may not be material since what it looks like it might be has been removed from newer models, that is not functioning up to specification? But, the front cat was bad and causing the car to get poor gas mileage so if I had done this first I wouldn't see the code anymore but my car would still be getting poor gas mileage.

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    Registered User pfsquirrel's Avatar
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    Quote Originally Posted by Donkey View Post
    Your wasting your money throwing parts it. The P0420 is for the rear O2 sensor only. If the car is completely stock I would have spent $15 on a mechanical CEL delete or this electrical one and called it a day.
    So this basically fools the O2 sensor into seeing the correct voltages, and resetting the mix back to normal?

    Out of curiosity, if the rear cat is indeed bad/going bad in terms of not going light-off due to carbon buildup or a bad substrate, does this workaround even make it activate? Wouldn't it potentially lead to carbon buildup? (not sure I worded that right, sounded right in my head)
    Last edited by pfsquirrel; 04-09-2013 at 10:44 AM.

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    Subaru Newb MainFrame's Avatar
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    Quote Originally Posted by ruckusman View Post
    Help me understand how this is not just hiding the problem or making the code go away. There is still a part, and the one now may not be material since what it looks like it might be has been removed from newer models, that is not functioning up to specification? But, the front cat was bad and causing the car to get poor gas mileage so if I had done this first I wouldn't see the code anymore but my car would still be getting poor gas mileage.

    The only function of the rear O2 sensor on your car is to throw a CEL or emissions purposes. Did you in fact determine that the front cat was bad? The pre-turbo catalytic converter should be removed regardless, also requiring a mechanical fix for the EGT sensor that would no longer be needed.

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    Subaru Newb MainFrame's Avatar
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    Quote Originally Posted by pfsquirrel View Post
    So this basically fools the O2 sensor into seeing the correct voltages, and resetting the mix back to normal?


    The rear O2 sensor has nothing to do with AFR.

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