Illumination of the Check Engine Light can be caused by a loose or worn gas cap.Google+
Car Problem Reports
GMC Yukon XL 1500 Check Engine Light Due to Gas Cap Issue
GMC Yukon XL 1500 Problem
Visitor, 2004 GMC Yukon XL 1500, 120,000 mi
Engine light code indicated loose gas cap - replace. I have replaced it 3 times parts store caps and oem caps. Still get light about every 6 months. When I open gas cap, gas tank is pressurized!