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Insulation hardened, cracked or melted by proximity to a heat source, so conductors are exposed or shorting. It is a slow failure with an abrupt end — the insulation embrittles over years and then cracks, so the fault appears suddenly on a harness that has been degrading since it was built.

Where it happens is predictable. Against an exhaust manifold, near a turbocharger, across a downpipe, or where a heat shield has corroded away or been left off. A harness that is stiff and shiny rather than flexible has been cooked, and it will crack wherever it is bent — including during the inspection.

The fault is usually a short to earth if the conductor reaches metal, or an intermittent short between adjacent wires inside the loom, which produces genuinely bizarre symptoms as one circuit drives another. Repair the whole affected length rather than the visible break, because insulation that has reached this state has done so along its whole exposed run — and refit or replace the missing heat shield, or the repair cooks too.