I had previously replaced all 4 spark plugs w/iridium plugs. On Friday 3/30/2012 I had all ignition coils replaced. The engine light came back on Sunday morning. Whatelse could cause cylinder l to misfire? Advance auto parts checked the engine light to see what it would say. I was told it was a misfire on cylinder l. Why is it still misfireing after all of the above?