Coil-on-Plug (COP)
An ignition system design where an individual ignition coil is mounted directly on top of each spark plug, eliminating spark plug wires.
Coil-on-Plug (COP) is a modern automotive ignition system design where each cylinder has its own individual ignition coil mounted directly on top of the spark plug. This eliminates the need for high-voltage spark plug wires, distributors, and remote coil packs. The engine control unit (ECU) triggers each coil electronically, ensuring extremely precise timing and a stronger spark for better combustion efficiency.
Because COP coils sit directly on top of the cylinder head, they are subjected to intense engine heat and vibration, which can break down their internal copper windings over time. Additionally, if the valve cover gasket leaks, oil can fill the spark plug wells, submerging the coil boots. This oil contamination dissolves the rubber boot and causes the high-voltage spark to short out against the engine block before reaching the spark plug.
Symptoms of a failing COP coil include engine misfires, rough idling, hesitation under load, poor fuel economy, and a flashing Check Engine Light. Mechanics diagnose COP failures by reading diagnostic trouble codes (such as P0300 to P0308), checking spark output with an ignition spark tester, or swapping the suspect coil to a different cylinder to see if the misfire follows it.