The spark plug ignites the fuel and air mixture in the engine's cylinder to power the engine. If the spark plug is defective, the engine may run roughly. To determine if the spark plug is defective, remove the spark plug and inspect it. If the porcelain insulator is cracked, an electrode has been burned away or damaged, or there is heavy carbon buildup at the electrode, replace the spark plug. Alternatively, if the spark plug hasn’t been replaced in a few seasons, replace it.