Car systems
Airbags and passive safety: how they work and what fails
Passive safety protects the occupants in a crash: the airbags inflate in milliseconds and the pretensioners tighten the seat belts. If the airbag light stays on, the system may not work in an accident, so have it checked soon and do not tamper with it.
Reviewed by Daniel Morales, founder of Reprorace SL, on
How it works
The airbag module receives the signals from the crash sensors spread around the body and from its own accelerometer. If the impact exceeds the threshold, it fires the gas generators of the relevant airbags and seat belt pretensioners.
Every time you start the car, the module checks the firing circuits and the occupancy and buckled-belt sensors, and lights the warning for a few seconds. If it measures a resistance out of range, a loose connection or a sensor that does not respond, it stores a code (B00xx) and leaves the light on.
The most common faults
- Connectors under the seats: they move when the seat slides or during cleaning; they are the most common cause of the light.
- Steering wheel clock spring: the ribbon that connects the steering wheel airbag breaks with use: the light and sometimes a horn or buttons that do not respond.
- Occupancy and seat belt sensors: the seat belt reminder does not go out or the passenger airbag switches off for no reason.
- Low voltage: with a weak battery, the module can store codes when starting.
- After a crash: deployed airbags or pretensioners are replaced; the module stores the crash data and often has to be replaced too.
A guide, most likely first: a repair shop confirms the diagnosis of each car with its own data.
Symptoms that can come from this system
Airbag warning light on: what it means and what to do
See a garage soon
Seat belt light will not go off: what it could be
Check without hurry
The most relevant trouble codes
- B0001Driver frontal stage 1 deployment control: malfunction
- B0002Driver frontal stage 2 deployment control: malfunction
- B0010Passenger frontal stage 1 deployment control: malfunction
- B0012Passenger frontal stage 3 deployment control: malfunction
- B0013Passenger knee bolster deployment control: malfunction
- B0020Left side airbag deployment control: malfunction
- B0021Left curtain deployment control 1: malfunction
- B0028Right side airbag deployment control: malfunction
- B0052Passenger seat belt sensor: malfunction
- B00D0Driver seat belt indicator: malfunction
- B00D1Passenger seat belt indicator: malfunction
- U0151Lost communication with restraints control module
Generic OBD-II/EOBD codes; each manufacturer adds its own. The same symptom can come from another system.
Code blocks of this system
Questions and articles about this system
- What does a pending, stored or permanent code mean?
- Can an OBD reader damage the car or drain the battery?
- Can I diagnose my car myself?
Learn more
See what your car is saying, system by system
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Frequently asked questions
Can I drive with the airbag light on?
The car drives, but the system may not protect you in a crash: have it checked soon. Do not unplug connectors or the battery to test it: an airbag can deploy.
Can the airbag code be cleared?
Only after finding and fixing the cause; if the fault is still there, the code comes back. Crash codes cannot be cleared: they require replacing parts.
Can an OBD reader read the airbags?
It depends on the car: the generic part mostly covers emissions, and the airbag module usually needs the manufacturer's diagnostics or a compatible tool.