How to Reset the Warning Light Gas Cap That Is Loose or Missingby Leonardo R. Grabkowski
Modern cars and trucks have an automotive diagnostic system known as OBD-II (on-board diagnostics). This system monitors a number of engine and emissions-related components to ensure that everything is in working order. If your gas cap is not properly attached, you may see a "Check Engine" light or a "Loose Cap" warning light on the dash. You can either tighten the cap and wait for the system to reset itself when it determines the problem is fixed, or you can use an OBD-II code scanner to manually reset the system.
Turn off your vehicle's engine. Push button to unlock the gas cap door, if your vehicle is equipped with a locking gas cap door. This is usually located on the floorboard next to the driver side seat, or beneath the steering column.
Step to the gas cap door. Open the door. Grasp the gas cap handle and twist it counterclockwise to unscrew it. Remove it.
Replace the gas cap. Removing and replacing it ensures the thread seals properly, in case it wasn't before. Turn the gas cap clockwise to tighten it. Continue turning until you hear it click three times. This ensures it's seating properly.
Close the gas cap door. Return to the cabin. Crank the engine and drive your vehicle for a day. The OBD-II system will automatically reset the "Loose Cap" or "Check Engine" light.
Use an OBD-II code scanner if the warning light does not disappear. Connect to the OBD-II port beneath the steering column. This port looks similar to a computer printer port. Push the "Reset" button on the code scanner to clear the system.
Continue driving the vehicle. Pay attention to the dashboard warning lights to ensure the light does not come back on. If it does, the gas cap may be faulty. Visit an automotive parts store or dealership service department to order a new gas cap.