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
Average Mileage: 81,625 mi (35,000 mi - 169,000 mi)
Visitor, 2001 Buick Century, 169,000 mi
gas cap hard to put on right , causes the check engine light to come on the next few miles
Visitor, 2000 Buick Century, 100,000 mi
NEVER overfill your gas tank. When filling up and the gas hose clicks off at the pump DO NOT fill to max. Also Make sure the gas cap is tight after filling so as not to create a vaccum. Common sense fixes this problem.
Visitor, 2003 Buick Century
I had to have the gas cap replaced because the light kept coming on and once the cap was changed no more problems.
Was in Florida and the light came on and when I got gas the cap was lose, tightened it up drove for a 1000 miles ant went off. Later same problem tightened it and light went off in a few miles.
got to click that gas cap at least 5 times when replacing it.