Catalytic Converter Code (P0420)

Discussion in 'General Motoring' started by allen.canterbury, Jan 23, 2006.

  1. Over Christmas, my CEL came on. I wasn't in a position to take it to a
    dealer, so I checked out my first theory a couple hours (and 100 miles)
    later. I'd had my oil changed a few days (and 200 miles) earlier, and
    I guessed they didn't put the air filter canister together right. I
    checked, and that was a problem - the canister wasn't put back on
    correctly. I put it back together and drove it another 70 miles and
    the CEL went out after about 2 drive cycles.

    A week or so later, the light came on again. This time, I took it to a
    local Advance Auto and had the code scanned - P0420 (catalyst
    efficiency below threshold). I took it to the dealer where they told
    me I needed a new cat - $1100 was the estimate. I figured I'd try
    replacing the O2 sensors first, incase they were marginal or starting
    to fail w/o throwing the O2 sensor code. The CEL went out by itself
    before I put the sensors on...put the sensors on anyway. I drove the
    car about 600 miles and the CEL came back on. Checked the code again
    and another P0420 showed up. After a couple days and a few short
    drives (40 miles or so total) the light went back out again and is
    still out.

    Anyone else had a similar experience? Any chance the incorrectly
    assembled filter canister could have somehow prompted this to happen??

    I'm buying an OBDII scanner on Ebay so I can keep it w/ the car and
    scan problems on-the-spot for the future, but I'd love to know what the
    code was the first time...anyway..

    If anyone has thoughts on this, please let me know. If I do put a new
    cat on the car, I'm going to put an aftermarket cat....probably a
    D.E.C. Anyone have thoughts or opinions on aftermarket cats (good
    idea/bad idea, problems, best choices?)

    allen.canterbury, Jan 23, 2006
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  2. allen.canterbury

    mulder Guest

    You didn't say what car you have, year and model, also if you are in
    the US. If in the US and your car is under 8 years and 80K miles,
    there is a federal emissions warranty that covers the catalytic
    conveter. As long as you haven't caused the failure by modifying the
    car the cat should be replaced by the dealer for free under this
    warranty even if your regular warranty has already expired.
    This code is almost always caused by the catalyst itself, and not the
    02 sensors. A bad 02 sensor will set its own code or codes, if all you
    got was the P0420 by itself it's most likely the cat.
    The loose airbox may have caused a different code, possibly a P0171
    lean condition code, but that would have nothing to do with the P0420
    or the catalyst. If the codes have not been cleared with a scanner and
    the battery has not been disconnected, it should still be possible to
    retrieve that first code by scanning for codes stored in the ECU
    mulder, Jan 23, 2006
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  3. Hi,

    On both 02' Legacy Outback and 02' Impreza 2.5 TS the P0420 comes up
    from time to time. Then it lasts for a few days and on the Outback
    vanishes where on the Impreza must be reset by the battery
    disconnection. It happens 3-5 times in the row and then it does not
    surface for months. It has been this way since about 40K miles on each
    car approximately.

    I did not notice any regularity there. Perhaps a colder whether is a
    factor: neither warm/hot nor dead cold - just colder.

    I was unaware ot that 80K warranty until read this thread. Thanks. I
    will take Outback to the dealer since it has got 70K on it but Impreza
    warranty is gone :-( (90K).

    Andy Leszczynski, Jan 26, 2006
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