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
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.