Capteur de choc du système de retenue défectueux

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.