Gatewaymodule defect

The module that connects the vehicle's separate networks to each other has genuinely failed — which is worth putting last on a suspect list rather than first, because a gateway originates almost nothing.

A vehicle runs several networks at different speeds: a fast one for the powertrain, a slower one for body electrics, a cheap single-wire network for switches and motors, and often a separate high-bandwidth one for entertainment. They cannot talk to each other directly, so every message crossing between them passes through the gateway. It repeats and translates; it rarely has an opinion of its own.

So a gateway blamed for invalid data is usually relaying invalid data faithfully, and the fault is on the far side of it. The genuine gateway fault has a different signature, and it is a change rather than a decay: a software update applied to one module and not this one, a module added or removed, a coding change, a retrofit.

Two things make a real gateway problem recognisable. Breadth — warnings appear in systems with nothing in common, which reads as a catastrophe and is one module. And the diagnostic socket, which on practically every vehicle built this century is wired to the gateway rather than to the buses. A tool that connects to some modules and not others, or times out on the whole vehicle, is the one symptom nothing else produces — and it is what makes the fault awkward, because the instrument that would explain it is speaking through the thing that is wrong.

One further consequence is the expensive one to miss. The gateway is normally among the modules deciding when the vehicle may sleep, so one that will not settle flattens a healthy battery in a few days.