Illumination of the Check Engine Light can be caused by a loose or worn gas cap.Google+
Car Problem Reports
GMC Yukon XL Denali Check Engine Light Due to Gas Cap Issue
GMC Yukon XL Denali Problem
Average Mileage: 71,000 mi (63,000 mi - 79,000 mi)
Visitor, 2005 GMC Yukon XL Denali, 63,000 mi
Check engine comes on. A bottle STP gas treatment or dry gas fixes it.
Check engine light due to gas cap issue.