▶FUNCTION◀-The fuel caps provides a tight seal to keep your vehicle's fuel tank closed, safety your vehicle's occupants, fuel economy and reduce vapor emissions. An old, loose gas cap, cracked or otherwise faulty gas cap may causes your check engine light CEL turns on. If your check engine light comes on or if you smell of gasoline or diesel vapors around or near the fuel filler cap and your car is not smoking or making a unusual noise, please take a minute to check the gas cap.