Generation IV Internal Engine - G8 GT problem

Hey guys the engine light comes on in the car when I get less than 1/2 tank of gas. Onstar said the gas cap was loose but it was not. I have tried using a little silicone grease on the gas cap gasket which didnt help. It only does it when you get below 1/2 tank. When you fill up it goes out after a few miles. Its still under warranty but was wondering if it was just something stupid I could fix.

have you put gas in when the car was still running?

have you put gas in when the car was still running?

no.
I have read about some evap problems with them but was thinkin about replacing the cap.

Go get a new gas cap. I am on my third one. same car same problem. its a fault in the design of the cap.