Ignition Coils and Coil-on-Plug

The ignition coil transforms the 12-volt battery voltage into the 20,000 to 100,000 volts needed to jump the spark plug gap. It does this through electromagnetic induction — the same principle that makes a transformer work. A primary winding of a few hundred turns of wire is wound around a core. A secondary winding of thousands of turns wraps around the primary. When the PCM interrupts current flow through the primary winding, the magnetic field collapses and induces a massive voltage spike in the secondary winding. That spike fires the spark plug.

Most modern engines mount an individual ignition coil directly on top of each spark plug. No spark plug wires. No distributor. Each coil fires only its own spark plug on command from the PCM. This gives the PCM individual control of ignition timing and energy for each cylinder. When a COP coil fails, only that cylinder misfires. The misfire code identifies the cylinder and the swap test confirms whether the coil is the fault.

A single-cylinder misfire at all engine speeds — the coil has failed completely on that cylinder. A misfire only under load or at high RPM — the coil is breaking down under demand. It may have a cracked housing that allows the spark to arc to ground instead of jumping the plug gap. Intermittent misfires that come and go — the coil may have an internal connection that opens when hot and reconnects when cool. Heat-related coil failures are extremely common. If one coil fails on a high-mileage engine, the others are likely not far behind.