A bad gas cap causes the “check engine” light to come on and does not hiss when unscrewed. It may also cause the car to smell like gas.
A bad, damaged or poorly fitting gas cap may not affect how the engine runs, but it may cause a fuel leak. It reduces fuel efficiency between 1 and 2 percent, and costs the car owner money. Most cars are designed to automatically detect fuel leaks using pressure tests to prevent air pollution. Pressure cannot build up in a fuel system if the gas cap is faulty, and this is what triggers the check engine light. Faulty gas caps should be replaced.