Illumination of the Check Engine Light can be caused by a loose or worn gas cap.Google+
Car Problem Reports
Buick Century Check Engine Light Due to Gas Cap Issue
Buick Century Problem
Model Year Affected: 1997
Average Mileage: 69,772 mi (41,000 mi - 98,544 mi)
2003 Buick Century code P0442. Evap system (small leak). Bought replacement fuel cap from GM (I read that it must be from GM). No "service engine light" after 50 miles.
Visitor, 1997 Buick Century, 41,000 mi
Replaced worn gas cap.