Check engine light comes on. Replaced original gas cap with a locking gas cap, light turns off and stays off. Can't stand the locking gas cap. I removed the gasket on the original gas cap, turned it around and put it back on the cap. Put the original cap back on. Light stayed off for about a week. Then it came back on. Tried the locking cap again, light would not go out. Played around with the gasket on the original cap and got the light to go out again. I am now looking for an OEM cap to replace the original with.
Gas cap had not been properly tightened to "locked" position, unbeknownst to me. My check engine light came on, and my awesome mechanic figured out the gas cap was causing it (without charging me a dime).