A Loose or worn gas cap may cause Check Engine Light to illuminate.
Car Problem Reports
Pontiac Torrent Check Engine Light Due to Gas Cap Issue
Pontiac Torrent Problem
Visitor, 2006 Pontiac Torrent, 50,000 mi
The gas cap wore out, and I replaced it for about $20.
Visitor, 2006 Pontiac Torrent
My Check Engine Light was going on, too, even though I'd securely tighten the gas cap after fueling. Even after I had the gas cap replaced, it'd occasionally go on. I just live with it.