Glow plugs are heating elements critical for cold-start operation in diesel engines. Positioned within the pre-combustion chamber or cylinder head, they rapidly heat the incoming air to facilitate combustion when engine temperatures are low. These metallic heating elements utilize electrical resistance to generate temperatures exceeding the diesel fuel's ignition point. Modern glow plugs commonly employ ceramic or metal sheathed heating elements for durability and faster heat-up times. Failure manifests as hard starting, excessive smoke, and potential misfire during cold operation; replacement is typically performed as a set to maintain balanced cylinder heating.