Dear Doctor: I recently had my 1999 Nissan Pathfinder serviced by a mechanic friend of mine. He changed the spark plugs and wires and did an oil change. Ever since then my service engine light has been going on and off. What could be the cause? Jim
Dear Jim: Anytime the check engine or service engine lights comes on, it indicates a signal or value that the computer cannot calculate. Before anyone can tell you what is causing the problem, the computer code needs to be retrieved. One of the biggest problems I see is that some technicians will get the fault code and just replace the parts in the system and not actually check the system before the parts are replaced. Just because there is a fault code does not mean the sensor in the circuit has failed.