To open the gas tank on a Cadillac CTS, push and release on the rearward center edge of the gas tank door. Ensure the car is unlocked as the gas tank is locked when the doors are locked. The vehicle has a capless refueling system, so you will not need to remove a fuel cap to fill the tank.
Be sure to remove the nozzle when it clicks off. Overfilling the fuel tank by more than three clicks of a standard fill may result in