Everyone knows that you’re supposed to shut the engine off when you’re filling your tank. Back in the day, this was especially important because most gas pumps didn’t have an automatic shutoff feature like they do today. It wasn’t uncommon to see someone have a geyser of gasoline shooting from the side of their car. Modern equipment is much safer, and spills happen less frequently. Even though the vapors are still incredibly explosive, because of the modern safety factor some people feel that it’s not as dangerous to refuel a vehicle with the engine running. It’s still a bad idea. It’s still dangerous, and even if you don’t cause a fire, your car’s electronics might just get confused and generate a “check engine” light. One of the reasons modern equipment is alot safer and cleaner than the older stuff is because it’s packed with electronics and sensors. Our modern cars are designed to run with a pressurized fuel tank and the car monitors this pressure in order to detect any leaks. It’s sensitive enough to detect any vapors that may be leaking from the tank. If your ignition is on when your gas cap is off, the car may sense that loss of pressure and BAM- you’ve got yourself a “check engine light”. Ain’t nobody got time for that!