A conventional diesel compresses air, then injects fuel into the resulting hot gas. The fuel ignites without a petrol-style spark plug. Glow plugs can provide additional heat for starting and other combustion conditions, depending on the system.

A difficult cold start or smoke does not identify a failed glow plug on its own. Fuel delivery, compression, temperature and other systems can contribute. Record conditions and obtain assessment rather than replacing the first plausible part.