trouble code P0421
 

Has anyone had a problem with a 99' protege (what I have), or anyhting similair, with the check engine light and a code of P0421? I am being told that it is either the pre-cat or an oxygen sensor. Both parts are only available thorugh mazda and the sensor costs 377 dollars! the pre-cat is 470 dollars. Before I make a purchase I want to know if anyone resolved this problem and what they did to fix it.Any help is greatly appreciated.

I had the same problem too, i have a pro 99 LX. I took it to the dealer, he said, just have the code clear and put in a fuel injector cleaner. that was like two month's ago. I haven't had the code since. Let me know, what you did to get it fix, good luck!

remedies tried
 

Plugs replaced with oem NGK....original standards in there for 100,000 miles...car has 101,???....what fuel injector cleaner did you use?...each time I clear code the time it takes to come back on is longer and longer...83 miles...168...350...I would love to find out if this code means bad o2sensor or bad cat...simply spoken.

info
 

BTW I have an ES 1.8 with 5 spd K&N box filter...all seems well with car except lots of rubber squeky sounds when cold...and that frickin light...AAGGHHHH!!!!!

usually if it's a bad o2 sensor, you can smell it. i'm not sure if the cat will give off that same smell in your exhaust fumes or not though. it's usually a fairly distinct smell.. not like normal exhaust. hard to explain though unless you know what it smells like i guess. try the fuel injector cleaner like someone else suggested to see if that helps it. get that BG44... my Mazda dealers both suggest it, because it's very strong, and clears all the carbon and shit out good. It's a little pricey, somethin around $20 a can.. but one can will last you a few months or something on higher mileage car, and for modded cars... i guess it just works awesome too. Try it.