B0004
Driver Knee Bolster Deployment Control
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Code B0004 indicates a malfunction in the driver knee bolster airbag deployment control circuit. The knee bolster airbag is designed to protect the driver's lower legs and knees during a frontal collision, reducing the risk of leg injuries by absorbing impact energy before the legs strike the lower dashboard or steering column. When this code sets, the airbag control module has detected an abnormality in the circuit controlling this device.
While you won't notice any change in how the vehicle drives, this code means your vehicle's occupant protection system is compromised. The airbag warning light will illuminate, and depending on the vehicle, the entire SRS system may be disabled — meaning other airbags may also not deploy in a crash. This significantly increases injury risk.
Diagnosis requires an SRS-capable scan tool to read sub-fault codes and pinpoint the exact circuit issue. Common culprits include corroded connector pins under the dashboard, pinched wires, or a failed knee bolster module. Because accidental airbag deployment can cause serious injury, this repair should always be performed by a trained technician who follows proper safety procedures, including disconnecting the battery and allowing the SRS capacitor to fully discharge before working on any airbag components.
Severity
Symptoms
- •Airbag warning light illuminated on dashboard
- •SRS system warning message on instrument cluster
- •Audible chime when starting the vehicle indicating a restraint system fault
- •No physical driving symptoms — risk is that the knee airbag will not deploy in a frontal collision
Likely Causes
The knee bolster airbag is located under the steering column. Its wiring harness runs through a tight area where it can become pinched, chafed, or corroded, especially if moisture enters the cabin through leaking seals.
The inflator squib in the knee bolster may have failed or drifted out of its expected resistance range, causing the airbag control module to flag a deployment loop fault.
The SRS control unit continuously monitors all deployment loops. An internal component failure or corrupted calibration data can cause it to report a false fault on the knee bolster circuit.
The SRS system requires stable power to function correctly. A corroded ground point or unstable ignition voltage can cause intermittent communication errors with the knee bolster deployment loop.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Inspect and repair wiring and connectors for the knee bolster airbag circuit
- Replace the driver knee bolster airbag module
- Repair or replace corroded ground connections
- Replace the airbag control module if internal fault is confirmed