Ignition systems have changed a great deal in recent years, with coil-on-plug (COP) ignition systems being the most common setup on many late-model engines. Car makers like COP ignition systems for a variety of reasons. The main one is that mounting a small ignition coil over each spark plug gets rid of the troublesome spark plug wires.

Plug wires are vulnerable to heat and vibration damage, and can become a source of ignition misfire if they become wet. Boots can loosen up as a result of mishandling (jerking on the plug wires), or become brittle and hard with age. Carbon tracks can also form inside the boots allowing arcing and misfires to occur. The insulation around old plug wires can break down and leak voltage, also causing the engine to misfire. In older carbon-core wire sets, the resistance of the conductor inside the cable can change as a result of heat and vibration, creating yet another source for misfires. So changing the ignition system to a coi...