gernby is right.... it could have temporarily cleared the code until it comes back, and you wont know what it is by clearing it (incase it was something you needed to solve) hopefully it was the gas cap thing and not something else. But if you get a check engine code, make sure you get it pulled before you clear it, because if there really is a problem just resetting the ECU will not solve it so you might as well leave the light on til it gets pulled.