The final reason why you may smell the scent of gas when you start up your car is because you have a gas leak. You may have a leak in your fuel line or you may have punctured your fuel tank. Instead, have your vehicle towed to an auto repair shop to have any damaged items causing the gas leak repaired.

Missing, Loose, or Cracked Gas Cap. The gas cap keeps gasoline vapors from escaping your fuel tank, so a cap that isn’t working right may just be the source of the smell. This is about the best-case scenario for a gas smell. If the gas cap is loose, simply screwing it in tighter should get rid of the smell.

A bad gas cap can allow fuel vapors to escape from the vehicle as it no longer seals the tank as it should. This will lead to the smell of gas around the outside of the vehicle and sometimes inside.

A faulty rail sensor may upset the air fuel ratio and cause the vehicle to experience performance issues such as a decrease in power, acceleration, and fuel efficiency, and in some cases may even experience stalling.

One of the most common symptoms of an issue with the gas cap is a cap that does not tighten properly. Most gas caps are designed to click once they are tightened sufficiently. If the cap does not click, or clicks and then pops loose again then that may be a sign that it may need to be replaced.

One common error code that you may encounter is “P0440” which indicates a loose gas cap. However, this is not the only code involving the fuel tank that could set off the “Check Engine” light. Many cars will also generate a warning signal if the on-board computer detects any fuel vapor escaping from the gas tank.

Therefore, a loose or a failing gas cap will bring about idling problems because of insufficient fuel system pressure. You can also have a rough idle from dirt and impurities that make their way into the gas tank’s neck because the gas cap cannot shut off correctly.

How to Reset the Warning Light Gas Cap That Is Loose or Missing

1. Turn off your vehicle’s engine.
2. Step to the gas cap door.
3. Replace the gas cap.
4. Close the gas cap door.
5. Use an OBD-II code scanner if the warning light does not disappear.
6. Continue driving the vehicle.

So first things first, pull over (as soon as it’s safe) and make sure your gas cap is on tight. If this was indeed the trigger, your check engine light should go off within 10 or 20 miles once you’re back on the road.