Well, seeing as neither the tank, or gas cap are malfunctions....obviously there is a malfunction with your engine or something else, and it needs addressed. Take it to a mechanic.
I drive an 04 Dodge Neon SXT. Why is my "malfunction indicator" light coming on?
It just came on today when I restarted my car AFTER I had accidentally turned it off with my keys while it was still in drive. I put in park, ran in to get what I forgot, came back out and that light was on.
It's not flashing.
My tank is full.
My gas cap is secure.
I'm just now noticing it and it's possible that it has been on but I really think I would have noticed it before.
I checked those two things because my car book told me it might be due to poor fuel or loose gas cap.