Faulty restraints crash sensor

One of the remote accelerometers that tell the restraints module an impact is happening. They are inputs to the decision, not devices that fire — which is what separates them from everything else in the airbag system.

They are mounted away from the module because of timing. A sensor in the middle of the car learns about a crash only once the structure has begun collapsing toward it, and a frontal airbag has about fifty milliseconds to be fully inflated — a side airbag has fifteen. So the sensors sit where the crash arrives first: forward in the crumple zone, and in the doors, sills and pillars.

Which is exactly why they are the restraint components ordinary events damage. A minor front-end knock, a bumper or door skin replaced, a kerbed sill, or water reaching a connector in a sill will all reach a crash sensor long before anything reaches an airbag. A car with repaired bodywork and an airbag light on is very often pointing here.

Not all of them are accelerometers. Many doors use a pressure sensor instead, reading the spike in air pressure inside the door cavity as the skin begins to deform — often faster than the shock travels through the structure. A code naming a numbered side sensor may be either, and only the vehicle's own documentation says which.

A fault does not disable one device; it disables the decision. With part of its picture missing, the module can no longer reliably tell a severe crash from a pothole, and normally shuts the whole restraint system down rather than risk being wrong in either direction.