A spark plug is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air mixture by an electric spark, while containing combustion pressure within the engine.
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 includes starting trouble, poor fuel economy, engine misfire, jerking, lack of acceleration etc.