B0099: Roll Over Sensor
What to do
Get this looked at promptly, and do not attempt it yourself. A restraint fault usually means the whole system has switched itself off, so the airbags and pretensioners may not fire in a crash — the car drives normally and is far less able to protect you. Never probe airbag wiring with a multimeter: the current an ordinary meter uses can set off the charge.
What this code means
The rollover sensor circuit is faulty.
A rollover is a fundamentally different crash from an impact, and it needs a fundamentally different sensor.
An impact is over in about a tenth of a second. A rollover takes one to three seconds — slow enough that an accelerometer looking for a sudden shock sees nothing worth acting on. So the module needs a gyroscope, measuring the rate at which the vehicle is rotating about its length, and it integrates that rate to predict whether the roll will continue past the point of no return.
That prediction is the hard part, because it has to be made while the car is still on its wheels and while 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 there to cushion a single blow — they are there to stay inflated for several seconds and keep occupants inside the vehicle, because ejection is what makes rollovers so lethal. A frontal bag deflates almost immediately; a rollover curtain must not.
So a rollover sensor fault removes protection in the crash type most likely to be survivable if the occupants stay in the car, and the vehicles most often fitted with one — tall, narrow, heavy — are the ones most likely to need it.
Likely causes
Start here
- Corroded or loose connector · specialist
- Faulty rollover sensor · specialist
- Damaged wiring harness · specialist
Less common
- Faulty restraints control module · specialist
- Water ingress into connector · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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