The fuel tank gas cap prevents gas or diesel vapors from escaping while the vehicle is in operation. By doing so, it reduces vapor emissions and aid to fuel economy. The gas cap on your Enclave, Escalade, Avalanche 1500, Silverado 3500 HD or Yukon XL 1500 does more than just closing the tank. If your vehicle is experiencing a Check Engine Light (CEL) and a code about EVAP leaks, gas cap wont click when closed or it is simply worn then you have to replace your gas tank cap.