A coil pack, also referred to as an ignition coil, is a component in the car's engine that changes current generated by the battery into a high-voltage current used by spark plugs. A typical coil pack transforms the 12 volts generated by the battery to the required voltage.
As of 2014, ignition coil packs are designed with either an internal resistor or connected to an external resistor. This controls the flow of current from the battery into the coil to ensure that the car's systems remain safe during ignition. Diesel cars typically do not have coil packs as they use a compression system for the initial engine ignition.