Sitzpositionssensor defekt

A switch or sensor on the seat rail or recline mechanism reporting where the seat is — how far forward it has been slid, and how far the backrest is reclined.

The restraints module uses it to soften what would otherwise be a full-force deployment. A seat slid fully forward puts the occupant much closer to the airbag, and that is usually a short driver who moved it up to reach the pedals — precisely the person a full-force bag is most likely to injure. Recline matters for a different reason: as the backrest goes back the hips slide forward relative to the lap belt, so in a crash the occupant can go under it, loading the abdomen instead of the pelvis.

It fails mechanically more often than electrically. It is usually a simple magnetic or Hall switch, and what stops it working is debris in the track, a bent rail, or a magnet knocked out of alignment when something was dragged out from under the seat. The wiring runs in the same flexing loom as the rest of the seat harness and is the other common answer.

Losing it rarely disables the airbag — the module falls back to assuming a normally seated occupant, which is safe for most people in most crashes and is exactly the assumption these sensors exist to stop it making.