A Loose or worn gas cap may cause Check Engine Light to illuminate.Google+
Car Problem Reports
Buick LaCrosse Check Engine Light Due to Gas Cap Issue
Buick LaCrosse Problem
Model Year Affected: 2005
Average Mileage: 72,496 mi (19,620 mi - 128,000 mi)
Visitor, 2005 Buick LaCrosse, 60,000 mi
Check engine light csame on, pulled trouble code it was stating gas cap problem replaced cap. turned out to be a bad sensor in the gas tank.
Visitor, 2005 Buick LaCrosse, 80,000 mi
In shop now for this and a power steering issue (I don't have details yet)
light is coming on occasionally even though cap is tight.