Did You Know: External Flashover


This bulletin will explain and visually show one failure mode of spark plugs adding confidence when replacement is needed.


  • All spark engine applications


Spark plugs can fail and will need replacement at their recommended mileage interval due to wear. But excessive idle time and constant short trips will shorten the spark plug life cycle due to fuel deposits or fouling due to oil, coolant, or chemicals. The shortened life cycle may also be caused by the spark plug not reaching self-cleaning temperature on a regular basis. The ceramic that surrounds the center electrode starts to accumulate deposits providing the voltage an easier path to ground causing external flashover and misfires.

If external flashover should occur, regardless of the sharpness of the electrodes, the spark plug will need to be replaced. The flashover will cause low power, poor fuel economy, increased tail pipe emissions and eventually misfires. External flashover will always require spark plug wire or coil boot replacement due to a carbon track, or trace, being burned into the boot.

Electricity will always take the path of least resistance to ground. Flashover, or commonly known as tracking, is when the electricity that is intended to travel through the spark plug travels the path of least resistance down the outside of the ceramic to ground. A smooth ceramic insulator spark plug is more susceptible to flashover than a ribbed insulator, therefore, smooth ceramic insulators are usually longer to help prevent this condition from occurring. When flashover occurs, a black mark will be etched into the exterior side of the spark plug and the inside of the coil or plug boot causing a misfire in the engine. This will be felt by the driver and will set a diagnostic trouble code in the powertrain control module. Flashover is the result of many conditions such as cracking of the spark plug ceramic insulator, moisture buildup in the plug boot, extremely large gap from a worn-out spark plug, poor connection at the terminal nut, extremely rich or lean air fuel ratio, or extreme high cylinder compression. If a misfire is felt and diagnosed to have tracking on the spark plug, the spark plug will need to be replaced. Not only is the tracking on the side of the spark plug but it will also be on the inside of the plug wire boot or coil boot. If the wire or boot is not replaced, even with a new spark plug, the electricity will have a path with less resistance to follow and damage the new spark plug. If the concern is not repaired in a timely fashion, this will cause larger issues such as damage to the coil.

In the below photos you can see what look like scratch marks in the spark plug insulator. If you study the images you will see that the marks on the spark plug match perfectly to the marks on the inside of the coil boot. Replacing just the spark plug is not a long-term solution, within hours of engine operation the voltage from the ignition coil will find that easier path to ground and in turn burn that same tracking into the new spark plug causing a repeat failure. Always inspect spark plugs when servicing for any unusual markings, replace the mating part to prevent a repeat failure.

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