Illumination of the Check Engine Light can be caused by a loose or worn gas cap.Google+
Car Problem Reports
Chevrolet Monte Carlo Check Engine Light Due to Gas Cap Issue
Chevrolet Monte Carlo Problem
Average Mileage: 115,000 mi (110,000 mi - 120,000 mi)
check engine light on for 2 years...changed gas cap 2 times..didnt fix issue
Visitor, 2001 Chevrolet Monte Carlo, 120,000 mi
If gas cap is not screwed back on tightly after getting gas, engine light comes on. After screwing on correctly and driving a while the light will go off. Can take a day or two for light to go off, unless a lot of driving is done.