I have a Pontiac G6 2008 4cyl and I just recently started having an issue with the car not accelerating like it use to. The CEL came on and indicated a P0420 code so the first thing I checked was my O2 sensors and ended up changing out the up-stream sensor, the downsteam was fine. I reset the code in the car and the CEL went off for about a day, but there was little improvement to my car. The next day the CEL came back on and the car seemed like it was worse. Took it to get the CEL code checked and the same P0420 code came up and the person told me it may be my catalytic converter. Since the catalytic converter is covered under warranty for 8 years or 80K miles I took it to the GM dealership and they said they couldn't figure out what was wrong with the car. He said everytime a code came up and he checked it the code would immediately clear itself out...basically it was misfiring. He said they hit the catalytic converter to see if there were any loose pieces or to see if they heard a hollow sound that would indicate it was clogged and they didn't get any indication that anything was wrong with the converter. I dont loose any gas mileage and i dont have a wet floor or the rotten egg smell that I have seen alot of people say they have had when theirs went bad. The dealership told me that since I have an after market alarm system (viper) I need to completely remove that and see if the problem is fixed. He said since the alarm system is hard wired into my cars computer it could be causing mis fires. Is there anyone who has had problems with their G6 like this or any insight on what the problem may be? This is starting to get expensive and it sucks that the dealership cant even figure out whats wrong but has no problem charging me and telling me they dont know whats going on!!!