Pontiac Repair: p0420 code on my 05 grand prix, o2 sensors, scan tool

Question
Hi Todd, I've had a p0420 code on my 05 base grand prix.  I've had catalytic converter changed, changed spark plugs and wires, had compression test. Spent 1200 already and about to spend more. No Mechanic knows how to shut that damned light off so I can get the car inspected.  About to have cat changed again with 02 sensors.  If this doesn't work, what else can I do?  Most mechanics don't want to touch my car.  By the way, it runs fine.  No misfires or trouble accelerating.

Answer
Hi Michael:

well first of depending on what stae your in the converter should have been covered under emission warranty of 8 year 80,000 or 7 70,000 depending on where it was bought. if the car has over that amount of milage then never mind it's too late now. if this car had an aftermarket convert put on it and not an OEM cat then that is the first place to look! aftermarket fits all parts do not work. I would get an original cat for it and start with just a post O2 sensor not both for right now. try that and see what happens. The engine control monitor looks at the pre and post o2 sensors to detrmine if the cat is working right or not. one other theing is the fuel trim is way off on this it will effect the way the cat works. You need to have it hooked up on a scan tool and see what the fuel trim rates are. my guess is they are relativly okay or else it would be setting rich or lean trim codes to go along with your 420.

good luck :-}