Its good to be back. It also means I have a problem. I got my cat changed out at 225,000. Low mileage at this point but the cat went bad. Changed it and went on my merry way with tons of power unlocked. Driving home yesterday and the mil comes on. I scan it and am aggravated as the text "1 code P0421" moves across the screen at 230,500. Ideas? There's no way it could have failed with 6k on it.

 

The catalytic converter is no longer functioning properly
An oxygen sensor is not reading (functioning) properly
A sparkplug is fouled up

That many miles id bet o2 sensor. Just replace both and be done with it.

 

I've replaced the front one at about 221,000. The rear one I can't replace as easily.

Why does it give that code? What are the 02 sensors reading? Or not reading? Difference in oxygen, temperature, how does it work exactly? I've also burn on average a quart over around 2,300 miles. I'm nervous that I'm ruining the new cat by it potentially dumping oil into it...idk. Spark plugs were changed the same time that the 02 sensor was replaced. FRONT o2 sensor.

 

It might be the 2nd sensor. Basically the second sensor checks to see if the first one is doing its job and if its not it will let the ecu know. Have you tried to add a cel eliminator to the second sensor? Might help if you didn't.

 

The 02 sensors are reading the percentage of oxygen that flows by them. The 2nd 02 sensor has to register a certain amount more oxygen passing by it than the first. (if there is cleaner air ie more oxygen passing by the 2nd sensor then the computer can determined that the cat is doing its job)

I still have both 02 sensors plus my WB and I have no cats but I also have no codes. Why? Because I used the antifouler trick to pull the 2nd 02 sensor further away from the exhaust stream giving it a reading of more oxygen thus tricking the ecu into thinking everything is hunky-dory

 

I had this issue with mine (199,xxx miles), and I put two 02 foulers on the rear 02 sensor, cleared the light, and it hasnt come back on yet. * fingers crossed *