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Thread: ODB-II Code P0421

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    ODB-II Code P0421

    I was wondering if anyone had any experience with the check engine light code P0421. I believe its either a bad catalytic converter or a bad downstream Oxygen Sensor.



    2001 Mazda Protege LX 2.0L 182,000mi

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    po420 is cat efficiancy below threshold.. 421 is also a cat code for bank 1 i believe in the case that you have 2 cats.. 1 dog and a turtle..

    anyway.. how many miles on the car? any missfires? or been a long time since a tuneup?

    more then likely its the cat isnt working right 2nd option is the DS o2 sensor 3rd option is to just get a obd1 car.. emissions suck ass

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    Quote Originally Posted by thtdudenamdbill View Post
    I was wondering if anyone had any experience with the check engine light code P0421. I believe its either a bad catalytic converter or a bad downstream Oxygen Sensor.



    2001 Mazda Protege LX 2.0L 182,000mi
    Had the same problem. It was the Pre-Cat. It was completely burned out due to bad ignition coils previously. I changed the O2 sensors prior to the Pre-cat hoping to save some money but to no avail. It was the Cat the whole time.

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    If you have had any ignition problem recently( timing belt/misfire/bad coils)then your pre-cat will need to be replaced. There are many ways to work around this problem. Install header, and or use the non-fouler trick. Be aware that these tricks may not be acceptable with your emissions inspection. BTW I also have the 2001 LX 2.0 with 180000 miles and I just replaced the pre-cat. I believe my cat failed because a mechanic installed my new timing belt a tooth off causing the ignition problem. Anyhow not that hard to replace if you do any simple mechanic work. Check out the thread: Tips for replacing pre-cat

    Hope this helps, let us know how it goes.

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    Oh, If your getting the P0421 code then I would say that the O2 sensor is doing exactly what it should be, reporting. That
    pretty much says that it is working. I believe this is the most common mistake made. "My O2 sensor is reporting. Well it must need replacing". WRONG!

    Also if you haven't resolved the ignition problem. Then don't expect your new cat to last very long.

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