Hitzeschaden an der Leitungsisolierung

Insulation hardened, cracked or melted by proximity to something hot — an exhaust manifold, a turbocharger, a heater element, or an EGR component. The damage is cumulative over years of heat cycling rather than a single event, which is why it appears on older vehicles and in the same handful of places on any given model.

It produces intermittent faults before permanent ones, and that sequence is the diagnostic clue. Cracked insulation conducts to earth only when it is damp, or only when the loom moves and the crack opens. A fault that comes and goes with the weather, or over bumps, on a wire that runs near something hot, is this.

Heat also causes it from the inside. A circuit carrying current through a marginal connection heats that joint, which degrades the insulation around it, which raises resistance further. Connector housings that look scorched or discoloured have been declaring this for months — so inspect for browning rather than only testing continuity.

Repair means replacing the damaged section and re-routing or re-sleeving it, not taping over it. Insulation that has hardened once will keep failing along its length, and heat shielding that has gone missing — a common casualty of earlier repairs — should be replaced with it.