I just got it serviced today because of the engine light. It's a little over 24,000 miles now. I've had this problem about 3 or 4 times since 5,000 miles. The mechanic said that the filter on the gas cap was dirty. It's been a pain in the ass. It gets a little sluggish also whenever the light is on.
Gas cap sounds like it locks, but you can pull it right off. Check engine light is constantly coming on. It's a pain in the butt. Have contacted OnStar with problem and have talked with their technicians about the problem. Last time we talked with them, we were given a document ID# and were told that it was an issue that needs to be taken up with our non-existent dealer. Will be making an appointment with dealership AFTER my company leaves!