A spark plug has a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator. The central electrode, which may contain a resistor, is connected by a heavily insulated wire to the output terminal of an ignition coil or magneto. The spark plug metal shell is screwed into the engine’s cylinder head and thus electrically grounded. Symptoms of spark plug failure include starting trouble, poor fuel economy, engine misfire, jerking, lack of acceleration, etc.