A gas cap is likely to fail when the seal breaks down over time and no longer seals to the top of the filler neck. It often won't click closed, it may have "play" when turning it, and you could experience a loss of fuel economy. Most gas caps are made from a durable plastic, but some are made from corrosion resistant metals. Usually a plastic one is best so there's no chance of corrosion or rusting.
The gas cap
seals the end of the filler neck tube to prevent harmful vapors from escaping the gas tank. There are some types of caps called locking caps. They include a lock cylinder embedded in the cap and a key used to open the lock cylinder. With a locking cap, a fuel thief won't have easy access to the fuel in your vehicle, and you can be sure no one will tamper with the fuel in your tank (ie. Putting dirt into the filler neck, fluids not intended to be combusted in the engine. This can cause significant damage to the vehicle including the rest of the fuel system and even engine components. The repairs for such damage are often way more expensive than the cost of upgrading to a locking fuel cap.
If your fuel cap is leaking or isn't situated properly onto the filler neck, you may see an orange colored "check engine" light appear in your instrument cluster beyond the top of the steering wheel. The first thing you should do is make sure you're in a safe place to pull over and open your door. Step out of the vehicle and walk to the location of your fuel filler door. Open the fuel filler door and take a look inside. Grab the fuel cap with the left hand or your right hand and jiggle the cap around to make sure it's locked nice and tight. If it's not nice and tight on there, unscrew the cap in a counter clockwise direction. Remove the cap and check for debris next to the seal that could be blocking it from making a tight connection. If there's debris in there, sweep it out with your finger or something small that can reach between the outer edge of the cap and the inner cylinder of the cap where the filler neck flange would sit when the cap is lock onto the filler neck. Gently place the cap onto the filler neck flange and turn clockwise until you hear a click. Turn a little more until you hear a second click and then the cap should be secure. Jiggle the cap again to be sure. If you are still experiencing a check engine light, the cap may need to be replaced altogether. This could happen because the cap is warped or the seal inside is old and brittle and can no longer seal tightly.
We have a wide array of gas caps for a multitude of vehicles that span generations. There are caps for small cars, large cars, trucks, SUVs, and even some medium duty vehicles. We carry some brands like Stant and Gates that are well known for their excellent fuel cap products. We usually meet or beat our competitor's prices and we strive to bring new products at low prices as often as we are able.