Illumination of the Check Engine Light can be caused by a loose or worn gas cap.Google+
Car Problem Reports
Buick Regal Check Engine Light Due to Gas Cap Issue
Buick Regal Problem
Model Year Affected: 2004
Visitor, 2004 Buick Regal, 178,688 mi
Check engine light came on got it scanned came up with the code meaning some type of emissions leak in fuel area. Was told it could be the gas cap. When I checked the gas cap it was worn. Bought a new one and in a day and half check engine light went away!