What it means
This code identifies a fault associated with the driver knee bolster airbag deployment control. The restraint control module sets it when it detects a problem in that deployment control; the exact fault is specified by the accompanying subfault information.
How it works
The driver knee bolster airbag is a restraint device mounted low in front of the driver, and its deployment control is monitored by the restraint control module. B0004 flags a fault in that control path; the accompanying subfault information is needed to tell whether the issue is an open, short, connection problem, or component fault.
Common causes
Most likely first.
- Knee bolster airbag wiring or connector fault
- Damaged deployment circuit
- Faulty knee bolster airbag module
- Restraint control module fault
Symptoms
- Airbag warning light on
- Knee bolster airbag may not deploy in a crash
How to diagnose it
- Read B0004’s subfault details and freeze-frame information with an SRS-capable scan tool before clearing anything.
- Follow the manufacturer’s SRS safeing procedure, then inspect the driver knee bolster airbag connector and nearby harness for looseness, damage, or corrosion.
- Check the knee bolster deployment circuit for continuity, shorts, and connector integrity using the manufacturer’s SRS test method.
- If wiring checks pass, test the driver knee bolster airbag module as specified, then assess the restraint control module only if the module circuit is sound.
- Clear the code after the repair and rescan to confirm the driver knee bolster fault stays cleared.
Common fixes
- Reconnect or repair a loose or damaged driver knee bolster airbag connector using the specified procedure.
- Repair damaged wiring in the knee bolster deployment circuit.
- Replace the driver knee bolster airbag module if it fails its specified test.
- Repair or replace the restraint control module if diagnostics confirm an internal fault.
Can I keep driving?
The airbag warning light indicates that the driver knee bolster airbag may not deploy in a crash. Drive only as needed until it is diagnosed, since this reduces restraint protection for the driver.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.