Circuit ouvert dans la boucle de déclenchement d'airbag

The circuit that would fire an airbag, pretensioner or load limiter is broken somewhere between the restraints module and the device.

A deployment loop is not an ordinary circuit and the module does not test it in an ordinary way. The device contains an initiator — a squib — with a precise, known resistance, and the module measures that resistance continuously. That lets it tell an intact device from a broken circuit without ever firing anything, which is the only safe way to check a one-shot explosive.

An open loop means the device cannot fire. And because a restraints module that cannot verify one loop will usually not authorise any deployment — firing some devices and not others can be worse than firing none — a single open circuit commonly disables the entire restraint system. The car drives perfectly and is far less able to protect anyone in it.

Where the break is depends on which device, and the pattern is worth knowing. For the driver airbag it is most often the clockspring, the coiled ribbon cable that carries the circuit into a steering wheel that rotates. For anything in a seat it is the harness beneath it, which flexes on every adjustment and is routinely damaged by objects pushed underneath or by seats being removed. For side and curtain devices it is usually a connector in the pillar or sill.

Never test a deployment loop with an ordinary multimeter. The current a meter passes to measure resistance can be enough to initiate a squib. This is what dedicated restraint-system equipment and the documented disable procedure exist for.