Capteur de retournement défectueux

A gyroscope that measures how fast the vehicle is rotating about its length, and predicts whether a roll will continue past the point of no return.

It is a gyroscope rather than an accelerometer because a rollover is not an impact. An impact is over in about a tenth of a second; a rollover takes one to three seconds — far too slow for a sensor looking for a sudden shock to notice. So the module measures rate of roll and integrates it, deciding while the car is still on its wheels.

That prediction is the hard part, because a violent avoidance manoeuvre must not be mistaken for a roll. A false deployment at speed would itself cause a crash.

What it fires is different too. Curtain airbags in a rollover are not cushioning a single blow — they stay inflated for several seconds to keep occupants inside the vehicle, because ejection is what makes rollovers so lethal. A frontal bag deflates almost immediately; a rollover curtain must not.

When it fails it is usually the module it lives in or the connector, since the sensor is typically integrated rather than separately serviceable. The vehicles most often fitted with one — tall, narrow, heavy — are the ones that most need it working.