Loose gas cap- Check engine light

My wife put gas in our 1998 Millenia a few days ago. It is now at 1/2 tank and the check engine light came on. I remembered reading something about the gas cap being loose will cause this. I checked and it was not tight at all. Could this be the problem?

It was at the dealer less than a month ago because of a check engine light and some emissions parts were replaced. It runs great with no problems at all. This forum is really great and is one reason I am not going to part with my Millenia!

Regards, Al