The coil pack is an essential part of the ignition of a vehicle. It consists of several coils in one fitting. Each coils role is to transfer the power from the vehicles 12volt battery in to a much higher voltage of thousands of volts to create the spark in spark plugs. They are then told when to fire by an on-board computer. These coils have some form of resistor in them, which limits the amount of power transmitted to the coils from the 12volt battery. This resistor can be an internal or external resistor or consist of a resistor wire. One coil pack is used for either one or two engine cylinders.
The downside of a coil pack is that if one of the individual coils in the pack goes, the whole pack needs to be replaced. Just one damaged coil can cause its adjoined engine cylinder to misfire. This may show on the dashboard as a ‘check engine’ light. Damage can be caused by worn spark plugs, which make the coil pack work harder to compensate, or faulty wiring or components. coil packs can also simply wear out through repeated use.