2005 Ford Mustang -CEL and gas cap

Twice now I’ve had the engine light and the gas cap light come on. I’ve learned that if I buy and install a new gas cap the engine light and the gas cap light turn off after driving the car for a few days. Why does the engine light come on for gas cap issues ?

The gas cap is part of emission control system. Any failure in the emission control system will trigger the CEL.


The gas cap is supposed to seal. Then the fumes from the gas tank can be managed back to the engine to be burned by the engine using the EVAP system. If there is an excessive air leak in the fuel system it will turn on the CEL. The computer runs tests periodically and checks for air leaks. It can determine the size of the leak. If the leaks size is around the size of a typical gas cap leak then the check has cap light comes on. If the gas cap is confirmed good, then the mechanic can trace where the actual leak is located.

Thank you so much. Best explanation of this issue. The Ford dealership wanted to run a test for $200.00.

Almost any shop will want to do a diagnostic before trying to fix this problem . An independent shop will be less expensive but make sure they really have the know how to solve this.

If you keep having gas cap issues, look at the sealing rim of the fuel neck. I had a Camry that kept throwing small evap leak codes and it turned out the lip of the filler was rusting. It only takes a pinhole there. One thing you could try is to apply some silicone grease to the gasket on the cap. If you don’t have silicone, try vaseline. If the codes go away, you know that’s the leak area. Wipe the vaseline off, it may degrade the seal long term although I left some on mine for years and no adverse consequences…

How certain are you the gas cap is even the problem? What DTCs are present?