An ignition coil is an induction coil in an automobile's ignition system that transforms the battery's low voltage to the thousands of volts needed to create an electric spark in the spark plugs to ignite the fuel. Some coils have an internal resistor, while others rely on a resistor wire or an external resistor to limit the current flowing into the coil from the car's 12-volt supply. In modern system. coils may be remotely mounted or they may be placed on top of the spark plug, known as direct ignition (DI) or coil-on-plug. Where one coil serves two spark plugs (in two cylinders), it is through the wasted spark system. In this arrangement, the coil generates two sparks per cycle to both cylinders. The fuel in the cylinder that is nearing the end of its compression stroke is ignited, whereas the spark in its companion that is nearing the end of its exhaust stroke has no effect. The wasted spark system is more reliable than a single coil system with a distributor and less expensive than coil-on-plug. Where coils are individually applied per cylinder, they may all be contained in a single molded block with multiple high-tension terminals. This is commonly called a coil-pack. Some of the symptoms of failing ignition coil includes engine misfiring, jerking, poor fuel economy, check engine lights on etc.