Hello, I'm wondering a quick cheap way to fix the two above codes. It's past due inspection. I've erased the codes and I'm hoping the check engine light won't go back on until I get it to pass inspection. The car runs fine.

 

I have a 1999 mazda protege. Check engine light came on. diagnostic code read p0304 replaced spark plugs light went off. (Feb.) Check engine light came on again in (May) code read p0421. Could you tell me what is it now? And how much will it cost me? single and broke need to sell right away, please help! saldbar

 

Misfires cause problems with catalytic converter. Change your PCV valve as well. Now that you hopefully got your misfire situation handled, it's time to do this cheap solution: