1996 Chevy Camaro Tank pressure sensor

I have a 96 Z28 camaro that the "service engine soon" light just came on. The code says that the Tank pressure switch is not purging, or senses the pressure wrong in the gas tank. Anyway I thought it was bad so I replaced it. The code cleared but I drove the car about 400 miles and it codes again. I was wondering if maybee the gas cap isn't sealing, or I have another sensor that went bad. If not what could be causing this code?