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A seat belt pretensioner, or its circuit, is out of specification.

A pretensioner is a pyrotechnic device, and this is the part of the restraint system people are most surprised to find contains an explosive charge. In a crash it pulls several centimetres of slack out of the belt within a few milliseconds, so the occupant begins decelerating with the car rather than a moment after it. Without it the belt does not begin working until the body has already started moving.

Its circuit is monitored exactly like an airbag's — by measuring the initiator's resistance continuously — so the same rules apply: an open loop disables it, and often disables the whole restraint system with it, and an ordinary multimeter must never be put across the circuit because the measuring current can fire the device.

The commonest cause is the wiring rather than the pretensioner, because the harness runs under a seat that moves. Objects pushed beneath the seat, and seats removed for trimming or cleaning and refitted carelessly, account for a large share of these faults.

Two things worth checking before a part is ordered. A pretensioner that has already fired reads as an open loop and is not visible from outside — belts fire in impacts too mild to deploy an airbag. And on belts with more than one pretensioner, the letter in the code identifies which one: a retractor pretensioner winds slack onto the reel, a buckle pretensioner pulls the stalk down, an anchor pretensioner tightens the lap section at the floor.