2004 Ford Excursion check engine light is on

My excursion is really a 6.8L V-10. About a week ago my check gas cap light came on and i checked to see if was loose it was tight and seemed to be no problem. A couple days later, my check engine light came on so I took it to Auto zone to get it checked out. The system read that it had a emissions leak. The guy told me it was my gas cap so i replaced my gas cap and the light still stays on. Can you diagnose my problem. Thanks so much for your help