I have an 04 gtp basically stock its had a p0420 code ever since i put a zzp dp on it with hf cat wondering if theres anything i can do like an 02 simulator or what not to get it off. but thats not really that big of a problem its the p0300 code that popped up recently so i just did a spark plug and wire change spark plug from cylindder 3 was broken at the tip. i replaced with napa beldan wires and autolite 606's. still showing the p0300 code for multiple random misfire. is it possible that with the p0420 code being on for so long it has ruined the o2 sensor and now its misfiring or is that a completely wrong direction and its possibly a coil or icm. any suggestions are appreciated
