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Thread: P0421 Code

  1. #1
    Registered Member


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    P0421 Code

    HELP. Getting this code on my 2002 protege 5,(P0421). 100,000 miles. Replaces plugs, replaces wires, both sets, whole egr valve (BRAND NEW), all vacuum hoses, O2 Sensors front and back (NEW). Erased check engine light went to smog and said FAILED because couldn't complete OBD self tests. Have to drive 80 - 100 miles and bring back. What gives. Any suggestions. Please don't say CAT. YIKES.

  2. #2
    Bassist butchblack's Avatar

    Mazda Protege5

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    Quote Originally Posted by themodefanatic View Post
    HELP. Getting this code on my 2002 protege 5,(P0421). 100,000 miles. Replaces plugs, replaces wires, both sets, whole egr valve (BRAND NEW), all vacuum hoses, O2 Sensors front and back (NEW). Erased check engine light went to smog and said FAILED because couldn't complete OBD self tests. Have to drive 80 - 100 miles and bring back. What gives. Any suggestions. Please don't say CAT. YIKES.
    Sorry

    The code is either the cat or the pre cat. If you're just at 100K you might want to speak to the dealer they may fully or partially cover it under emissions warranty if your engine is stock.

    Mine does the same thing @ 115k so I'm expecting a 500-1000 bill next time I go for inspection. I might wait till after I fail as I can get an exemption in Mass if I spend enough on trying to fix it, in case there is other problems, but I like the car so I may just get it done right.

    Does anyone know if this is a common problem on 2002 Protege5's?

    Butch


    Butch

  3. #3
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    yea, you need a cat

  4. #4
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    which one there are two??

  5. #5
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    the one right off the manifold.

  6. #6
    Backfires destroy MAF's!


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    ^ or replace with a header if your state is not crazy-strict on emissions...

    or

    Try doing the CEL eliminator (search) for your 2nd o2 sensor. I did, and it got rid of my CEL for now. I know my 1st cat is plugged, I'll replace it with a header when I get paid for my overtime...

  7. #7
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    2002 Protege5 @102,000mi-- Same issue: P0421 CEL and a puff of smoke when i start it after a day of sitting. Trying to put off replacing the front cat, but worried about damage being done to the back cat as a result. Seems to be a trend with P5's...

  8. #8
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    2002 Protege5 @102,000mi-- Same issue: P0421 CEL and a puff of smoke when i start it after a day of sitting. Trying to put off replacing the front cat, but worried about damage being done to the back cat as a result. Seems to be a trend with P5's...

  9. #9
    Riddle me that! slavrenz's Avatar

    2001 Mazda Protege LX 2.0

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    Quote Originally Posted by riri2469 View Post
    2002 Protege5 @102,000mi-- Same issue: P0421 CEL and a puff of smoke when i start it after a day of sitting. Trying to put off replacing the front cat, but worried about damage being done to the back cat as a result. Seems to be a trend with P5's...
    This does happen a lot with the P5's unfortunately. And the code won't trip if the rear cat isn't working, since only the one near the exhaust manifold is monitored by the O2 sensors.

    RockAuto.com FTW - Walker exhaust sells a new pre-cat for about $260.

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