Approximately 90 percent of all plug failures are caused by carbon fouling. During the combustion of the air/fuel mixture, carbon deposits can build up on the firing end of the insulator nose. As these deposits accumulate, a conductive path is formed from the center electrode, down the insulator nose to where the insulator meets the metal shell. This buildup provides a path for the electrical current to leak through. When voltage is applied, under certain conditions, the carbon path may absorb enough current to prevent sufficient voltage to build up at the gap, and misfire occurs.
Possible causes: Rich fuel mixture. If the fuel mixture is rich, a complete burn is not achieved, which can result in carbon being deposited on the insulator nose. Due to this excess fuel, the insulator nose is kept at a lower temperature, so the carbon has less chance of being burnt off.