Pesky P0420 Code on an '04 WRX
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This is a discussion on Pesky P0420 Code on an '04 WRX within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; Alright, I got the intermittent CEL for the pesky P0420 Catalyst System Efficiency Below Threshold (Bank 1). Generally, I can ...

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    Pesky P0420 Code on an '04 WRX

    Alright, I got the intermittent CEL for the pesky P0420 Catalyst System Efficiency Below Threshold (Bank 1). Generally, I can reset the CEL code and continue on; however, I think that I have a bad oxygen sensor. The sensor is operating at 100.2% efficiency...enough to throw me a code.
    There are no performance issues when driving; however, when started I can smell gasoline vapors when the engine's cold and only exhaust fumes when warm.

    I've been reading through SEVERAL postings on this and cannot figure out if my culprit is the front or rear mounted o2 sensor. The car only has 62K miles, so my thought is a front sensor. Does anyone have any suggestions before I crawl under to checkout the situation??

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    Registered User 06scoobyrex's Avatar
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    po420 means that the car got too high... lol
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    Good news, everyone! xxxxxxxAnub1s's Avatar
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    Known to be an issue with the rear O2 sensor. have you done anything different with your car lately? Fuel additives, downpipe?
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    Registered User speedracer866's Avatar
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    Bank 1 is the front O2 sensor bro, a common problem with subarus. I recently changed mine as well. Although the rear can go bad highly unlikely as the front one is what tells the car if to richen or lean out.
    Last edited by speedracer866; 04-06-2010 at 03:17 AM. Reason: Update Of Response
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    Good news, everyone! xxxxxxxAnub1s's Avatar
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    Quote Originally Posted by speedracer866 View Post
    Bank 1 is the front O2 sensor bro, a common problem with subarus. I recently changed mine as well. Although the rear can go bad highly unlikely as the front one is what tells the car if to richen or lean out.
    P0420 is "Catalyst system efficiency below threshold". Bank 1 isn't specified. The computer is simply noticing that there is no difference between air into and out of the cat, which indicates either the cat has gone bad, which is why I asked about fuel additives or down pipe (high flow cat, or no cat), or the rear O2 sensor has gone bad (unlikely as there should be other codes as well). The front O2 sensor would throw a different code (of which there are many).

    If he recently replaced his down pipe, this error would be the first thing I would expect.
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    He simply abides. SD_GR's Avatar
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    Quote Originally Posted by speedracer866 View Post
    Bank 1 is the front O2 sensor bro, a common problem with subarus.
    There is only one bank. It therefore does not specify physical location. The code number itself is used for that.
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    Registered User speedracer866's Avatar
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    Quote Originally Posted by SD_GR View Post
    There is only one bank. It therefore does not specify physical location. The code number itself is used for that.
    Ok I am sure I read in here something about bank 1. They are two banks 1 and 2, 1 being the front O2 sensor and 2 being the rear. Anub1s is correct with the trouble code description, however I have known a bad O2 sensor to cause problems around this area.

    I have also read articles online where some owners had this problem changed the cat and rear O2 sensor and problem still came back.

    However I suppose the best course of action would be to deal with the code you are getting first. Keep me posted on your progress.
    Last edited by speedracer866; 04-06-2010 at 03:34 PM. Reason: Update
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    He simply abides. SD_GR's Avatar
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    Quote Originally Posted by speedracer866 View Post
    Ok I am sure I read in here something about bank 1. They are two banks 1 and 2, 1 being the front O2 sensor and 2 being the rear.
    There is only 1 bank.
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    Registered User speedracer866's Avatar
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    Quote Originally Posted by SD_GR View Post
    There is only 1 bank.
    No quarrels here dude but I have seen this with my own eyes. Anyways our dispute of banks isn't helping with the problem.
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    He simply abides. SD_GR's Avatar
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    I'd urge the OP to check the rear O2 sensor including the connection and wiring.

    Also see:
    o2 sensor
    if error is Bank 1 sensor 1 vs. Bank 1 sensor 2 etc.

    EDIT: Somewhere there is a picture thread but I am sorry I cannot find it. If I do I will update.
    Last edited by SD_GR; 04-06-2010 at 03:50 PM.
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    Just replaced the rear 02 sensor and the CEL came on again...the code reads as a P0420 (same as last time) in Bank 1. I failed to verify whether it was Sensor 1 or 2, but with a car that only has 62K on the odometer how often do oxygen sensors really go out? Went and bought 2 anti-foulers for the rear 02, prolly install within a few days and update progress.

    On a side note...I've also got a DP that deletes the cat (not installed yet) that includes the rear bung for the 02 sensor. Has anyone had any problems with CEL codes if they are not using the Cobb AP or revised maps?

    Unfortunately, I have to deal with Maryland emissions restrictions and I really hate jumping through these damn hurdles.

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    Good news, everyone! xxxxxxxAnub1s's Avatar
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    If the cat in the downpipe has been deleted, this code will be thrown, no matter what. You can get the mechanical fix for the O2 sensor which basically pulls it further out of the exhaust stream, or you can get some form of tuning, as any stage 2 map should be ignoring this code anyways.

    On an 04 WRX, you shouldn't be running the DP without engine management of some kind or another anyways.
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    Registered User speedracer866's Avatar
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    Which sensor did you change. If it was the rear first you shoudl change the front one. That is the one that sends the signal to the computer whether to richen or lean out.
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