Alright so i have a 2005 pontiac grand prix GTP (3.8l v6 supercharged) 4 gear automatic. It has 113,000 miles on it and i have a p0420 code. When my car is warmed up and under load it misfires. I can really only press my throttle down about 30-40% until it starts misfiring, although just cruising low throttle it runs perfect. I have brand new spark plugs and 02 sensors. I have no clue what could be causing my misfiring as it could be many different things, it could even be my gas it reccommends 91 octane but it says 87 is fine just less performance so i fill it with 87 octane (im going to see if this problem persists with 91 though). A while ago i had a maf sensor code pop up so i sprayed it with cleaner and that code hasnt popped up for 4 months since i did it. Lastly i do believe i have a stock catalytic converter, its not clogged because i still get all the speed. The only thing that limits my speed is the misfiring. Now idont drive my car like a race car i just want to know what the problem could be incase its harmful.
Forgot to say it has a clean throttle body, and 0 vaccum leaks, engine misfires on cylinder 3 the most but will also so it on 2 and 4 sometimes, no visible gas leak. Clean air filter as well.
check the FAQ section of this forum for diagnosis.
Already did that, posted info trying to find actual help, not for someone to just tell me to read an faq.
trying to find actual help, not for someone to just tell me to read an faq.
Why not? We're just going to tell you to do exactly what's in the FAQ.
Why do you think it's there? The FAQ contains the basic stuff you need to eliminate first, and it solves 99% of cases.
When you've exhausted the simple stuff, then we would happy to help you find the cause.
