PO421 - What should I be fixing? (Warm up cat efficencey code)?

I have a 2005 Mazda 3, 2.3L, Automatic, with 160,000km on it.

For a couple months it would throw code PO421 (Warm up Catalytic efficiency below thresh-hold, Bank 1). (A couple days on, a couple days off), when the light is on I have noticed a bit of a dip in my fuel economy, BUT, that could very well be a coincidence.

I replaced my pre-cat (under the car) as per advice of my mechanic, reset the light, and it came back a week later. It is doing the exact same thing, on/off, on/off. I am thinking about replacing both my 02 sensors, but would rather not waste money if anyone else has any suggestions.. (It starts great all the time so I dont think it is a bad Spark Plug).

3 Answers

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  • 10 years ago
    Favorite Answer

    Its most likely your cat, its an 05 so if it were your O2 sensors you would get a code for it or them, the light coming on and off doesnt mean the problem goes away :check engine light" are designed to come on and off to protect light bulb life but still needs to be fixed accordingly.

    Source(s): 9 year fuel inj tech.
  • 10 years ago

    I would replace the forward O2 sensor - no need to replace the rear one. The front one is going to be sluggish after 160K km and that may make the difference. It is interesting you don't have a P0420 code, which means the system knows how to behave once it warms up.

    The forward O2 sensor is worth replacing on the basis of improved fuel efficiency over the next 5-10 years and it has a decent chance of fixing the P0421.

    Source(s): 35 years maintaining my own cars
  • Anonymous
    10 years ago

    your cat works as a buffer for the air to exit the engine at the right pace. if the flow of air is to slow/fast etc it will always determine your fuel efficiency. it might be the cat its self that is the issue if you haven't had it changed and its done 160k it could be shot to bits inside.

    if your able take it off and shake it if it rattles then you need a new one. or you can take it to the garage and they can test the cat by the amount of gas that exits the exaust. it may even be clogged after that milage.

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