After running my codes at autozone, it said I had a bad o2 sensor (bank 2, rear sensor). I replaced it myself, and went back to have the SEL reset and codes erased. The service engine light stayed off for about a day, and now it is back on with the same code. The car runs great when the SEL is off (had it reset twice) but the second its back on, the car is a dddoogggggg.

Any ideas???

 

p0420? If so it's most likely a bad catalytic converter.. In short: The rear 02 sensor isn't suppossed to fluctuate like the front. When it does it sets that code indicating the cat isn't doing it's job... Comes with a fed mandated 8yr/80k mile warranty..

That code shouldn't cause a poor running condition.. Maybe that code is setting due to the fact the car is running poorly sending unburnt fuel into the exhaust... Any other codes? My guess is that you have another problem sending unburnt fuel into the exhaust which took out the cat or the cat is clogged(unlikely due to the intermittant symptoms)