Illumination of the Check Engine Light can be caused by a loose or worn gas cap.Google+
Car Problem Reports
Chevrolet Impala Check Engine Light Due to Gas Cap Issue
Chevrolet Impala Problem
Average Mileage: 137,752 mi (100,000 mi - 163,258 mi)
Visitor, 2004 Chevrolet Impala
have replaced the gas cap 3 x at different intervals so far.
Visitor, 2005 Chevrolet Impala
The gasket around inside cap had become hard and develop cracks allowing pressurize air to leak out causing the check engine light to turn on. the code P0442 was reported "small leak detected"
Visitor, 2004 Chevrolet Impala, 150,000 mi
The gas cap was loose and wouldn't tighten, causing it to fail inspection.