My service engine light came on, so I brought it in to my mechanic. He says my gas cap is causing the light to come on, (this was told to me by the secretary). The gas cap was put on right, I always make sure I tighten it until it clicks three times. They did not suggest to me what I need to do to solve this problem. i did ask the secretary if i should change the gas cap and she said yes. Should I replace the gas cap? My car has also been idling high and low ever since this happened, is this the cause of a faulty gas cap?