The gas cap or the fuel filler cap is a crucial piece of stuff that prevents the dirt and debris from entering the gas tank. The gap cap and the reliable seal it provides- wear out over time and need to be replaced. You can’t tighten a faulty gas cap. You will experience a fuel smell inside. The illuminated check engine light can be the another indication of a bad fuel filler cap. The replacement of a gas cap is absolutely easy. Take the old- damaged one off before putting the new. Still, for assistance, if you want you can call a car professional.