I replaced all 4 spark plugs w/iridium plugs. I also replaced all 4 ignition coils on Friday 3/30/2012. The engine light came back on on Sunday morning. After I did all the above, why would the engine light come back on and show a misfire on cylinder l? I would like to donate at least $3. I don't have a credit card. All I have is a debit card.