I have a Pontiac GrandAm 2002 and the light service engine soon came on. Would that be caused by a bad gas cap? The car seems to be running fine. This light came on about 6 months ago and the dealer wanted to charge me 1200. to replace a gasket. I took to another mechanic and they looked up the codes I had gotten from the dealer and they replaced the themostat now it's back on. Do have any other suggestions?