I have a 2003 Protege ES. A day after I had a P0300 code problem fixed (new coil and EGR), my check engine light came on. This time the code is P0421, “Cat below threshold (bank1)”. From reading the forum, it appears to be an O2 sensor or the catalytic converter.

Is there a way to determine which part is bad? Also does “bank 1” refer to just the front cat and O2 sensor or could the rear ones be at fault also? By the way, the car runs great, no smells or noticeable misfires and power is at normal levels.

Jim
Minnesota

 

i have had the same problem happen to me. just like everybody i decided to replace the cheapest parts which were the 2 O2 sensors but my CEL was still on. although the O2 are cheaper, they are $150 - $200 for each straight from the dealer and unfortunately it didnt fix anything. so i figured that it was the cat but i still wasnt sure which one it was, so i went to my local mazda dealership and the service person was nice enough to let me know that it was the pre-cat with just me letting him what the code was. so i bought a new pre-cat and the CEL surprisingly went away. hopefully this answers your question.

 

Thanks, that is what i thought. I'll post back once I put the new cat new.