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The two conductors of a data bus pair are touching each other.

This is the most decisive fault on any vehicle network and the easiest to confirm, because of what the pair is doing. The two wires carry the same signal inverted — one rises as the other falls — and a receiver reads the difference between them. Joining them makes each cancel the other exactly. There is no difference left to read anywhere on the network, so it does not degrade: it stops.

And the measurement is unmistakable. A healthy pair reads its network's terminated value across the two conductors; a shorted one reads close to zero. That reading is wrong on every bus there is, whatever its speed, medium or normal figure — which makes it the one bus measurement that needs no manufacturer data at all.

Two wires twisted together for their whole length only touch for physical reasons, and the list is genuinely short:

• A crush. A loom trapped by a panel, a bracket, a seat bolt, a boot-floor fixing or a jacking point.
• A fastener through the harness. A screw fitted during a repair, a trim clip, a self-tapper from an accessory installation.
• Water bridging a connector. Rarely a dead short on its own, and enough to stop the bus — which is why a network fault appearing after heavy rain or a flooded footwell is looked for at a connector rather than along a wire.
• Rodent damage, where the insulation is gone from both conductors at one point.

The presenting picture is worth recognising: a vehicle that appears completely dead electronically while its lights, horn and starter still work, because none of those is on the bus. A car that cranks but will not run, with a dead dashboard and a scan tool that cannot connect, is describing this almost exactly.

And one free check belongs on any towing vehicle: unplug the trailer and inspect the socket. A socket full of water, where the bus runs nearby, is a recognised cause.