Dear Tom,
I own a 2002 Ford Focus. The other day I filled it with gas and now the gas cap is gone and the check engine light is on. I must have put it down at the gas station and left it there. Did I damage my car? Why is the light on?
Cynthia from Baton Rouge, LA

No, your car is not damaged, Cynthia. Go to your local Ford dealer and get a new gas cap. Install it and keep an eye on the check engine light for a few days. It should go off after about 100 miles of driving. If it doesn't, take your car to a shop so they can scan for codes.

Cynthia asks a popular question. What happens if my gas cap is left off? Why does it cause the check engine light to come on? Have I done some damage to the engine? Let's explore these concerns.

Can leaving the gas cap off harm my car? No, aside from an annoying check engine light from a false EVAP System failure code.

Why can the absence of a simple gas cap cause the check engine light to illuminate? The fuel system in your car is a sealed system that traps harmful gas fumes. If the gas cap is left off gas is able to escape from the system. When this happens, a self-test of the performance system detects this "leak" in the EVAP (Evaporative Emissions System) and the check engine light comes on, and a code is stored.

Can I fix this myself or do I have to go to a shop to fix it? In most cases, simply replacing the gas cap and driving the vehicle for 100 miles or so will allow the system to reset itself. If this doesn't work and the light stays on, then there are other complications; the system should be scanned and the problem diagnosed.

If the gas cap is left off, can the gas leak out of the tank and cause a problem? Is this dangerous? No. A flapper valve is built into today's fuel filling systems. This valve stops reverse flow of fuel out of the tank. If you stood the vehicle on its head, gas would not flow out of the tank because the flapper valve is an anti-roll valve. So it's not dangerous to leave the gas cap off. However, you will be able to smell gas fumes. And I suppose, if you were mindless enough to stick your head close to the open gas tank filler-neck with a lit cigarette and provide an ignition source for the escaping fumes, you could be compromising your health.

Where do I buy a new gas cap? I would buy an OEM (Original Equipment Manufacturer) gas cap because it will always thread correctly into the filler neck, the seal will seal properly, the proper vent valves will be in place and the cap will be designed specifically for your vehicle's fuel system. Deviate from OEM design and you are asking for trouble.

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