The top reason ignition coils fail is bad spark plugs or plug wires. Another potential cause is heat and vibration, which can damage the ignition coil's windings and insulation. If an engine is experiencing repeated coil failures, the underlying cause may be resistance from worn spark plugs or excessive spark plug gap. In rare cases, the failure may be due to a lean fuel condition caused by leaky valves.
An automobile's ignition coil provides the high voltage needed by the ignition system to fire the spark plugs. It increases the ignition system's primary voltage from 12 volts up to thousands of volts. If a spark plug or plug wire is open or has excessive resistance, the ignition coil's output voltage can shoot higher and burns through the coil's internal insulation, causing a short. This is most likely to occur when output exceeds 35,000 volts.
When a coil failure occurs, the coil's voltage output drops, and the engine may not start or may misfire badly when under load. The misfire may also jump from cylinder to cylinder. A car owner suspecting damage to the ignition coil should measure the coil's primary and secondary resistance with an ohmmeter. If either is out of specifications, the coil needs to be replaced.