I have 2006 Grand Prix 3.8 N/A with aprox 116,000 miles that the P0420 code keeps popping up however everything appears to be fine according to my local shop leaving me with a what to do now question. Recently I had some other work done so I am not sure if this could have caused the problem. The car basically was missing out due to a faulty coil pack and ignition module. I was leaving a friends house late at night when this problem popped up so I turned around and parked it until I could have it picked up. I had to much stuff going on to deal with it at the time so the GP was towed to be fixed. When I started it to get the car into possition for the tow truck the check engine light was no longer flashing however the exhaust reaked of sulfter. I know the coil and module would have been an easy 20 min fix in a parking lot but I had to much going on at the time to deal with it so almost $500.00 later from the dealer (after a family discount) I got the car back and it was fine for about a week. I was leaving work just after it had rained and the car next to me was trying to get in front of me when there was plenty of room behind to slip in. I always have thought this is very rude expecially when the car you are trying to get in front of is already going above the posted speed limits. So anyways I decided to floor it resulting in my traction control to come on cutting power and causing the engine to rev enough for the rev limiter to kick in. So I backed off and then started in again...same thing happened. From that point I backed off and said screw it...the car next to me (follow GP) was having the same problem. Probably about 5 mins later is when the check engine light came on. I have had the code cleared several times, cleaned the throttle body, MAF sensor, fuel treatment, etc but it always comes back in about 2 days. Also over the past few weeks my fuel economy has really started to drop. My mechanic said that I have plenty of power however another thing I have noticed is that the transmission is less willing to shift into the next gear 2nd to 3rd, 3rd to fourth which makes me think it is a preasure issue with the cat.
At this point I will end up having to take it some place else to pin point the problem but I am curious about what else could be throwing the code. I know 02 sensors however they seemed to check out. Also, I have heard various things about using an aftermarket cat vs a GM cat. What is the general take on it or does anyone have any suggestions?
Thanks
