I have new plugs & wires - put in at the same time as the head gasket. The engine doesn't seem to misfire at all. it runs really well. Do you think the o2 sensor would cause the engine light to come on only at certain times & then go off though? i know thats s very common issue and not too serious - but in the past when I've had o2 sensor problems, the check engine light had come on and stayed on.