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
Average Mileage: 145,500 mi (112,000 mi - 179,000 mi)
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.