This glow plug provides the heat source needed for cold-start ignition in a diesel engine. It is installed in the cylinder head, where its tip extends into the pre-combustion chamber or direct-injection bowl. When the ignition key is turned to the preheat position, the plug reaches operating temperature to raise the air-fuel mixture above the cold-start threshold. A failed glow plug produces hard starting, white exhaust smoke, or a misfire during cold cranking. This component is manufactured to the original electrical resistance and heat-up profile so that the engine control unit's preheat timing remains accurate. On replacement, clean the bore and apply anti-seize compound to the threads.