Illumination of the Check Engine Light can be caused by a loose or worn gas cap.Google+
Car Problem Reports
GMC Sierra 2500 Check Engine Light Due to Gas Cap Issue
GMC Sierra 2500 Problem
Visitor, 2003 GMC Sierra 2500, 179,000 mi
Emmisiions light on for months, was told to live with it, I brought it in for other work and they replaced the gas cap...light is off....hmmm go figure.