Thread: 2004 GTP Code P0420

Originally Posted by Gohan77

Hi everyone. New to the forum but not to Grand Prix's. This is my third one and so far my favorite. However I got a code today of P0420 twice. I have a custom exhaust (SLP Loudmouth exhaust 2.5") but i believe the cat was left there. Is it the o2 sensors or the cat itself? Hopefully it's the sensors, but I can live with getting a new cat; i guess it could be a lot worse than that. Ideas?

Google spark plug fouler trick..do that and see if the code continues to appear.