Illumination of the Check Engine Light can be caused by a loose or worn gas cap.Google+
Car Problem Reports
Pontiac Bonneville Check Engine Light Due to Gas Cap Issue
Pontiac Bonneville Problem
Average Mileage: 144,000 mi (118,000 mi - 189,000 mi)
Visitor, 2003 Pontiac Bonneville
When I put my gas cap on, I have to turn it past being on. I turn it until it clicks several times. Otherwise, my service engine light comes on.