What Does B0004 Mean?
This code indicates a malfunction in the driver knee bolster airbag deployment circuit or its control system. The Restraint Control Module (RCM) has detected a subfault condition affecting the knee bolster airbag's ability to deploy properly during a collision. This is a critical safety system fault that compromises occupant protection in frontal crashes.
Safety Warning
It is NOT safe to continue driving with this code as the driver knee bolster airbag will not deploy in a collision, significantly increasing risk of leg and lower body injuries. This requires immediate professional diagnosis and repair due to the life-safety nature of airbag systems and liability concerns.
Common Causes
35%
Damaged or corroded wiring harness and connectors in the knee bolster airbag circuit
30%
Faulty knee bolster airbag squib or inflator module
20%
Failed Restraint Control Module (RCM) internal circuit or driver
10%
Open or high resistance in airbag deployment circuit due to previous accident or repair
5%
Low vehicle battery voltage or poor RCM power/ground connections
Diagnostic Steps
1
Step 1: Disconnect the negative battery cable and wait 10 minutes for airbag system capacitors to discharge. Verify battery voltage is above 12.4V before reconnecting for testing to eliminate low voltage as a cause.
2
Step 2: Visually inspect the knee bolster airbag connector under the dashboard for corrosion, pushed-back pins, or damage. Check the clock spring connector if the circuit passes through the steering column.
3
Step 3: Using a digital multimeter, measure resistance across the knee bolster airbag squib terminals with the airbag disconnected (typical specification: 2-3 ohms). Verify circuit continuity from RCM connector to airbag connector.
4
Step 4: Check for proper ground continuity at the RCM and airbag circuit ground points using a multimeter (should be less than 0.5 ohms to chassis ground).
5
Step 5: If wiring and squib resistance are within specifications, substitute a known-good Restraint Control Module or perform manufacturer-specific RCM self-tests to isolate internal control module failure.
6
Step 6: Clear codes, perform a comprehensive airbag system test using the scan tool, and verify the airbag warning light extinguishes and no codes return after repair.
Estimated Repair Cost
$200 - $1,500
Parts + labor, varies by vehicle and location
This code indicates a malfunction in the driver knee bolster airbag deployment circuit or its control system. The Restraint Control Module (RCM) has detected a subfault condition affecting the knee bolster airbag's ability to deploy properly during a collision. This is a critical safety system fault...
The most common cause of B0004 (Driver Knee Bolster Deployment Control (Subfault)) is: Damaged or corroded wiring harness and connectors in the knee bolster airbag circuit
Typical repair costs for B0004 range from $200 to $1,500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
It is NOT safe to continue driving with this code as the driver knee bolster airbag will not deploy in a collision, significantly increasing risk of leg and lower body injuries. This requires immediate professional diagnosis and repair due to the life-safety nature of airbag systems and liability concerns.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B0004 to identify the root cause.
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Quick Info
Category
Body
System
Supplemental Restraint System (SRS/Airbag)
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
A quality scan tool helps you read codes, view live data, and clear faults.
