The coil pack assembly is an essential part of the ignition of a vehicle. It consists of several coils in one fitting and is connected to a minimum of four engine cylinders. 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.

The downside of a coil pack assembly is that if one of the individual coils in the assembly 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 assembly work harder to compensate, or faulty wiring or components. Coil pack assemblies can also simply wear out through repeated use.