Sometimes, if you park a vehicle on a slope with the front end downward a "vacuum pressure" in the gas tank will cause the check engine light to come on. SOMEtimes, merely taking off and replacing the gas cap will clear it. If not, at my Ford dealership, it costs $68 + 13% tax to clear it... or $10 cash for my buddy at my service station. He never wants the money but I tell him if he doesn't take it I will drop it on the ground and walk away and that would be rude so, "Please don't embarass me."