Restraint device already deployed and not replaced

An airbag, pretensioner or load limiter has already fired and has not been renewed, so its loop now reads as open.

Every device in a restraint system is single-use. Once fired, the initiator is spent and the loop reads open for ever — which is exactly what the module reports, and why this cause looks identical to a wiring fault until somebody looks at the part.

On a car with a visibly deployed airbag this is obvious. The case that matters is the one where it is not. Seat belt pretensioners fire in impacts well below the threshold that deploys an airbag, and a fired pretensioner is not visible: the belt simply feels tight or will not retract properly. A car repaired after a moderate front-end knock can have had its bodywork put right and its pretensioners left spent.

Which makes this a used-car finding as much as a repair one. An airbag lamp on a vehicle with a suspiciously tidy front end, or a belt that does not retract smoothly, is worth taking seriously — and it means the car is being driven with no functioning restraint system, which no inspection of the paintwork will reveal.

There is a harder version. Deployed devices are sometimes replaced with a resistor to extinguish the lamp rather than with a new part. That leaves a car that reports itself healthy and will protect nobody, and it is worth a scan-tool check of the restraint module's crash record before buying any vehicle whose history is unclear.