Yesterday, the little idiot light that is in the shape of an engine came on in my wife's car.  I looked at the owners manual, it said very little, except sometimes the gas cap not being on properly might cause this check engine light to come on.  She did find the cap loose, she said, and I put it on correctly, but the light is still on.

Anyone have a clue what else that light might mean?  The car seems to run and drive fine, it is just frustrating to see the light on.

Thanks, John