Does the engine seem to he shaking or feel like a miss is in it? If it does, have the codes read and see which cylinder is missing. You really need to have the codes read anyway. Most Auto Parts houses (Autozone, Advanced Auto, Pep Boys) will read the codes for free. Once the code is set, it will stay in the system for a long time. Please post back with the exact code number (P0304 or P0420). The codes will be like the numbers in parenthesis. Do not post what the person reading the code says it is. We need the number.