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Resistance where there should be almost none, in a circuit carrying a very large current — the intake air heater, the fuel supply heater or a glow plug feed, all of which draw tens of amperes.

This is the fault that ordinary testing misses, and understanding why is the whole point. A multimeter passes about a milliamp, and a corroded or loosened joint conducts that perfectly — so the circuit measures fine while collapsing under the current it actually has to carry. The right test is a voltage drop measurement taken while the heater is working, not a continuity or resistance check at rest.

It is also self-accelerating, which is why a marginal joint does not stay marginal. Resistance in a high-current path dissipates real power at that point, and the heat oxidises the joint further, raising the resistance again. A connection found discoloured, or with softened insulation around it, is the fault rather than a symptom of one — and it is on a path toward burning through.

Where it concentrates is predictable: relay contacts pitted by switching, heavy cable terminals that have relaxed with heat cycling, and the stud on the heater itself — which sits on the inlet manifold, looks like part of the casting rather than part of a loom, and is the connection people walk past.

The symptom is a heater that half works: a diesel that starts but cranks longer and smokes more on a cold morning, with everything measuring intact.