Airbag light on: reading the B code, what can be cleared and what cannot

What you notice

The little seated figure with a circle in front of it stays on after starting, when it should go off after a few seconds. Sometimes it flashes, sometimes it only comes on when you adjust the seat or turn the steering wheel. The car drives normally, and that is precisely the trap: the restraint system (airbags and seat belt pretensioners) may be disabled, partly or completely.

Why an ordinary OBD2 reader sees nothing

The airbag light is controlled by the airbag control unit (often called SRS or ACU), generally installed in the middle of the car, under the centre console. Its faults are coded with the letter B (body), often in the B00xx range for restraint systems, with manufacturer codes as well. Mandatory OBD2 only covers the engine and emissions: an adapter that only queries the engine control unit will tell you “no codes” while the airbag light is on. It is not the adapter that is faulty, it is the wrong door.

With DiagAuto AI, the scan on the “Control units” screen looks for the vehicle's other control units and shows which ones report faults, and how many. If the airbag control unit answers and is compatible, the app reads its codes, without purchase; it tells you what it was actually able to read on your car. Some makes do not allow this control unit to be read with an ELM327 adapter: the manufacturer's tool is then needed. The app does not clear the codes of the other control units and does not change anything in the car.

The most common causes

1. Clock spring (the flexible ribbon under the steering wheel that connects the driver's airbag): it wears or breaks from repeated turning of the wheel. Associated signs: horn or steering wheel controls no longer working, a code referring to the resistance of the driver's airbag.
2. Connector under a front seat (side airbag, pretensioner): it works loose or corrodes when the seat is moved, when vacuuming or after removal. A classic on many models from the 2000s and 2010s, including Renaults and Peugeots. A light that comes on when you move the seat is a strong clue.
3. Seat belt buckle or passenger presence sensor: worn internal contact, faulty occupancy mat in the passenger seat.
4. Voltage too low: a tired battery that collapses when starting can make the control unit store a fault. If the code refers to supply voltage, look at the battery first.
5. Recent work: seat removed, steering wheel taken off, dashboard work done with the ignition on. The fault stays stored even once everything has been reconnected.
6. After a crash: airbags or pretensioners deployed, or simply a recorded crash. This is the only case in which the control unit keeps data that cannot be cleared.

What can be cleared and what cannot

• An ordinary fault (connector, clock spring, voltage): once the cause is repaired, the code often switches to “stored” and, on many cars, the light stays on until it is cleared with a diagnostic tool that can access the airbag control unit. This is normal and is not proof that the repair failed.
• A fault that is still present: it comes back immediately after clearing. Clearing without repairing is pointless.
• Crash data: after a deployment, the control unit keeps it. An ordinary tool cannot clear it. Depending on the manufacturer, the control unit is replaced by a professional, along with the deployed components.

Disconnecting the battery changes none of this: the airbag control unit's memory does not depend on power. For the engine control unit, see also what clearing codes really does.

What you can do right now

1. Note when the light comes on: all the time, after moving the seat, when turning the steering wheel, only when starting in cold weather.
2. Check the battery if the car is hard to start: a low voltage sometimes explains everything.
3. Run the control unit scan in the app to find out whether the airbag control unit answers and whether it reports a fault. Note the code if it can be read, then look it up in the fault code dictionary.
4. Do not take anything apart yourself: not the steering wheel, not an airbag, not the control unit. Never measure an airbag or a pretensioner with a multimeter: the meter's current can be enough to set it off. Simply checking a connector under the seat is done, according to manufacturer procedures, with the ignition off, the battery disconnected and after a waiting time: if in any doubt, leave it to a professional.

When to go to the garage right away

• The light stays on all the time: assume that protection may be missing in a crash, for you and your passengers.
• After an accident, even a minor one, or if an airbag has deployed.
• If a child travels in the front in a rear-facing seat: the passenger airbag deactivation system must work beyond any doubt.
• Before a roadworthiness test: a lit airbag warning light leads to a re-test in France. See Roadworthiness test: check your codes beforehand.

Indicative cost

Actual prices depend a great deal on the model: always ask for the code that was read before accepting a quote.