Faulty restraints control module

The airbag control module — the unit that decides, in a few milliseconds, whether a crash is happening and which restraints to deploy.

It should be low on any suspect list, because the things around it fail far more often. Its satellite crash sensors, the seat occupant classification sensing, the seatbelt buckle switches and the connectors under seats and sills are all commoner — and the corpus's standing point applies with force: restraints wiring is disturbed by an enormous range of unrelated interior work.

Water is the leading physical cause, because the module is typically mounted low in the centre console or under the carpet — which is exactly where a leaking screen seal, a blocked drain or a spilt drink ends up. A damp carpet with an airbag light on is a strong signal.

One property makes it different from every other module on the vehicle, and it is worth knowing before a repair is discussed: many of these modules record a crash event permanently, and lock themselves once deployed. A module from a vehicle that has had airbags fire generally cannot simply be reset, and a second-hand one may arrive already locked — which is why replacements are usually new and usually dealer-programmed.

The safety framing overrides the diagnosis. An airbag warning light means the restraint system cannot guarantee it will work as designed, and nothing about driving the car conveys any urgency at all — which is precisely why it gets deferred and should not be. Work on these circuits is for somebody equipped for it; they are pyrotechnic devices, and this is not a place for casual probing.