I have a light on that says "check gas cap" and the service engine soon light is also on. Don't know why--just started today. I don't drive the car much anymore--only 2 days a week to work on Tuesday and Thursday.
The onboard diagnostic system gave me a check radiator cap notice. I check the cap and it was screwed on firmly. Apparently the cap is getting worn and it intermittently compromises the "Closed system". I have not had to replace the cap yet and the light has not come on in quite awhile. Until it comes on and stays on, I'll keep the old cap.