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The fuse or fusible link feeding the glow plug system, blown so no heating occurs at all. Glow plug circuits carry very high current — often over a hundred amps across a bank of plugs — so they are protected by a heavy fuse or link rather than a blade fuse, and that protection is doing a real job.

A blown fuse is a symptom, and replacing it without asking why is the mistake. These fuses do not fail from age; they fail because a plug has shorted internally, a supply cable has chafed to earth, or the control module's output stage has failed short. Fitting a new fuse to a shorted plug blows it again immediately, and repeatedly doing so damages the wiring the fuse exists to protect.

Measure each plug's resistance before replacing the fuse. A healthy plug reads well under two ohms; a shorted one reads near zero and is the outlier in the set. The complaint is a hard cold start with white smoke and no fault when warm, since the circuit is only used below a temperature threshold — which is also why the fault seems to disappear for the summer.