Technician A says a black line on the ceramic end of a spark plug is normal.
Technician B says the line could indicate a misfiring spark plug.
Technician B is correct.
The most common failure in COP ignitions is carbon tracking or “flash over” on the spark plug insulator.
Although carbon tracking is usually caused by oil, dirt or moisture creating a path to ground over the spark plug insulator, a severely eroded spark plug electrode will increase firing voltages to the point that the spark will seek the point of least resistance, which is either through the spark plug boot or down the insulator to the metal shell.
If a carbon track is found on a spark plug, an identical track will be found inside the spark plug boot.
If the bootis serviced as a replacement part separate from the coil, the boots should be replaced at the same intervals as the spark plugs.
If the boot is available only with the coil assembly, the spark plugs should be replaced at the recommended intervals and care should be taken not to contaminate the boot or insulator with dirt or oil.
Always make sure you inspect the old spark plugs and boots before you install the new plugs. This video is sponsored by Autolite.