Thread: 01 pontiac grand prix code po420!

i have a 01 grand prix 3.8l the only code i have on the car is po420. the cat was replaced two times since we have had the car brand new. the MAF has also been replaced. both 02 sensors have been changed in the past two years. when i replaced the front 02 sensor the code po420 had popped up. with an almost brand new cat on the car, what else could be cauing this code? an exhaust leak was fixed where the muffler and tailpipe meet, which i wouldnt think would cause this code. anything else that could cause this?

i have erased the code multiple times and it always comes back the most recent time it took almost 150 miles to pop back up.

there was a stock cat put on the first time, then my father was told that it was defective when he had the code come back on. he got another one and over two years later the code is back. the rear 02 sensor was replaced in 09, the car was not running for about a year in 2011, and a new front 02 sensor was replaced just within the last month. could that cause this code?

the car had a blown head gasket which is what caused it to be off the road for so long. could this have causes the cat to fail? cant seem to find any exhaust leaks. i had somebody remove the rear 02 sensor as was told it drive sluggish when removed(got this idea from a forum). there is no rattling sound coming from the cat, as well as no damage done to the sensors. anybody know what i can do? car has 113,000+ miles on it.

2001 pontiac grand prix, 3.8l
code po420 popped up after changing the upstream 02 sensor. no rattle from cat, nor no sign of damage. was just replaced within the last few years. just replaced the upstream 02 sensor about a month ago, and the downstream was replaced about 3 years ago. the car had a blown head gasket, whic caused it to be off of the road for over a year. what could be causing this code to reoccur? i need to pass delaware inspection on jan 10.

How was the rear O2 sensor "removed"? Was it tuned out or just physically removed and capped off? If the latter, then you need that O2 back in place. How did it blow a head gasket? These motors rarely if ever eat HG's. Usually the LIM gaskets go, or the UIM cracks if your car is not supercharged. Typically this code means the cat is toast.

This means that the secondary O2 sensor detects that the catalytic converter isn't working as efficiently as it should.
This engine code can be triggered by any one of the following:

1.) The catalyst in the catalytic converter is cracked, clogged, or otherwise damaged.
2.) The secondary O2 sensor is malfunctioning.
3.) There is an exhaust leak.
4.) The computer unit is faulty.

seems to be no problem with sensors, cat, and no exhaust leaks found. the code spit up after replacing the front 02 sensor.