Airbags

B0004 — Driver Knee Bolster Deployment Control Circuit Fault

Severity: high Drive with care, to a shop

What B0004 means

The restraints control module (RCM) self-tests the driver knee airbag circuit and found a problem such as an open, short or out-of-range resistance. Normally it monitors the igniter circuit continuously so it can fire it in a crash. The "subfault" label is a manufacturer-specific sub-code detailing the exact failure type.

Symptoms

  • Airbag (SRS) warning light on in the dash
  • Stored driver knee airbag circuit fault in the RCM
  • Light may stay on or flicker intermittently
  • Part of the restraint system may be disabled by the module

Possible causes

  1. Open or shorted wiring to the driver knee airbag
  2. Poor connection or corrosion at the connector under the steering column
  3. Faulty knee airbag module (igniter)
  4. Harness or connector damaged during interior work or trim removal
  5. Faulty restraints control module (RCM)

How to diagnose

  1. Scan all SRS codes with a capable scan tool and note any related faults
  2. Inspect the harness and connectors under the dash and at the knee airbag for damage or corrosion
  3. Confirm connectors are fully latched, especially after recent interior work
  4. Check battery voltage and ground connections
  5. Clear the code and see whether it returns immediately or when the harness is moved
  6. Measure circuit resistance only per the manufacturer procedure with proper tools; never probe an airbag circuit with a regular meter
  7. Test or replace the RCM if wiring and module check out

Common fixes

  • Repair or replace damaged wiring and connectors
  • Replace the driver knee airbag module
  • Replace or reprogram the restraints control module
  • Clean corroded terminals and fix the cause of moisture

Questions and answers

Can I test an airbag circuit myself?

It is risky. A meter or static discharge can set off the igniter. Disconnect the battery and wait the time stated in the service manual, or leave it to a shop.

What should I do if the airbag light is on with B0004?

Get the SRS system diagnosed soon. Until it is fixed, the knee airbag, and possibly others, may not deploy in a crash.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.