Cat Convert or 02 sensor on Mazda 3?

I am throwing code PO421 Warm Up Catalyst Efficiency Below Threshold (Bank 1), my mechanic though it was my Cat Converter (underneath) so I got it replaced but it is still throwing the code. (Comes on and off randomy), since it is still throwing, maybe it is a 02 sensor?

3 Answers

  • Anonymous
    1 decade ago
    Favorite Answer

    It means that th two O2 sensors, one before and one after the cat are not showing the proper split in readings.

    Is it winter where you live and it happens in the AM at 0 deg F and not on a 40 deg F morning??

    My bet is on the Forward O2 sensor, but have to watch them with my computer to be positive.

    ASE Cert Auto Tech, since 1978

    2010 Mazda 3 5dr Sport 6 spd MT

  • Anonymous
    5 years ago

    If you follow the link you will see what the senor looks like off of the shelf. So I do not believe that it is causing the oil light to come on. I have put a link below to the Mazda 3 forum about this sensor, you might want to join it or at least check out the other repair info there.

  • Steven
    Lv 4
    1 decade ago

    could be one or both O2 sensors, wouldnt hurt anything to change those. Sometimes aftermarket catalytic converters arent as good as ones from the dealership, and could still cause problems

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