Loose or Faulty Gas Cap
You wouldn’t think a gas cap would be that important, but it is. When it’s loose or cracked, fuel vapors leak out and can throw the whole fuel system off. This causes a reduction in gas mileage and increases emissions.
What causes it: If you get an error pointing to the gas cap it means fuel vapors are leaking out of your cap. This means the cap is either cracked or just wasn’t tightened well enough.
What you should do: If your car isn’t feeling jerky or strange when the check engine light comes on the first you should check is the gas cap. Pull over, retighten it, and take a look at the cap to see if it has any cracks in it. Continue driving and see if the check engine light turns off. Alternately, you can purchase a gas cap for about $3 at an auto parts store. All you need to do is take the old one off and screw on the new one. If you’ve already made it to the store, you might as well just replace it. While not car-threatening, it’s good to take care of this right away to improve gas mileage.