B0004
Driver Knee Bolster Deployment Control.
- Generic
- Body
- Generic code
- High severity
How this code is built
- B
- SystemBody
- 0
- Defined bySAE, the same on every car
- 0
- AreaBody area 0
- 04
- FaultThe specific fault in that area
What it means
B0004 is the generic code for a fault in the driver knee bolster (knee airbag) deployment control circuit. The airbag control unit runs a self-test on the deployment circuit, or squib, and has seen a problem such as an open circuit, a short, or a resistance outside the expected range. Exact circuit details and whether the car has a knee airbag vary by make and model, so check the manufacturer's information for the vehicle.
Symptoms
- Airbag or SRS warning light on the dashboard
- Warning light stays on after start-up rather than going out after the self-check
- Driver knee airbag may not deploy in a collision, though this cannot be seen from the driving seat
- Intermittent SRS light, often after the steering column or lower dash has been moved or worked on
- Other restraint codes stored alongside it
Possible causes
- Poor connection or damaged connector at the driver knee airbag module or its under-dash wiringlikely
- Damaged or chafed wiring in the lower dash or steering column area, causing an open circuit or shortlikely
- Faulty driver knee airbag module or its initiator (squib) with out-of-range resistancepossible
- Disturbed or incorrectly refitted trim, knee panel or connector after previous repair or accident workpossible
- Corrosion or moisture in a connector, for example from a water leak into the footwellpossible
- Fault inside the airbag control unit, such as a damaged deployment driver outputrare
- Previous crash damage or a module that has been deployed and not replacedrare
How to diagnose
- Read all stored and pending codes and any freeze frame data. Note whether B0004 is current or history, and look for related restraint codes.
- Ask about recent work or accident damage to the dash, steering column or lower trim, and check for water ingress in the driver footwell.
- Make sure the battery is disconnected and the manufacturer's waiting time has passed before touching any airbag component. Never probe an airbag circuit with a meter or test lamp.
- Visually inspect the driver knee airbag module, its connector and the wiring behind the lower dash for damage, pinching, corrosion or loose connectors. Check any yellow restraint connectors are fully seated and locked.
- Inspect the connectors for bent or pushed-back pins, and check any shorting clips or connector position assurance locks are intact.
- Using the manufacturer's procedure, check the circuit resistance and live data for the deployment loop with approved equipment. Test values and method depend on the model.
- Where the procedure allows, substitute a known-good module or a correct test load to decide between a wiring fault and a faulty module.
- After repair, clear the code using the correct procedure and confirm the SRS light goes out and the code does not return.
How to fix
- Repair or replace damaged wiring, and refit connectors securely, following the manufacturer's repair guidance for restraint circuits.
- Clean or replace corroded connectors, and fix the source of any water ingress.
- Replace the faulty driver knee airbag module if the circuit or the module is out of specification.
- Refit any displaced trim or panels correctly so wiring is not trapped or stretched.
- Repair or replace the airbag control unit only after wiring and module faults have been ruled out, and follow the manufacturer's coding or setup requirements.
- Replace any deployed or crash-damaged restraint components as a set, as the manufacturer specifies.
Can I keep driving?
High severity
The car can usually be driven to a workshop, but the knee airbag may not deploy in a crash, so it should be treated as a safety fault. Have it inspected promptly and avoid unnecessary journeys until it is repaired.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.