Kay, i have a 2002 cavalier. the code PO420 came up and I have already replaced the cat. and i have had the o2 sensors monitored. the rear one wasnt working so i replaced it. but the code came up again. i was told it could also be my temperature gage, so i checked that but its working perfectly fine. so is there anything else it could be? i'm trying to avoid going to a garage. what else should i be looking at or checking? anyone know?

 

Your temperature guage doesn't have anything to do with that code. More than likely it's either going to be the cat or an o2 sensor. I would try replacing the first o2 sensor and see what happens. You could have possibly gotten a bad converter, or maybe you have a leak in front of the converter?

 

Instead of wasting money on a possible bad front o2 sensor, go to an autozone or advanced auto parts. They will read the code for free and tell you exactly what needs to be replaced.

 

Your problem is probably that you used an aftermarket O2 sensor. Post cat sensors must be replaced with OEM sensors form the dealer
The rear O2 is designed to be a bit lazy and the aftermarket ones work too fast.

I've had the exact problem with the exact car once and fixed it by ordering a sensor from the local dealer