2012 Chevrolet Impala engine light and gas cap

Recently had a problem with "tighten gas cap" msg appearing and then the engine light would come on. We had gas cap replaced, and it seemed to be alright. About a week ago, it started happening again. Now the check engine light is staying on, check gas cap msg rarely appears. The car is running fine, nothing obvious that we can tell. Our mechanic says if it continues we'd need to take the car to the dealership for diagnosis and repair. Being on a fixed income, we are hoping someone out there might have a solution that would help. Dealership repair is so far out of our budget.