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Severity: High Code type: Generic (all makes)

B0004 Code: Driver Knee Bolster Deployment Control

Fault in the firing circuit of the driver's knee airbag in the lower dash below the steering column.

Safety first · Consult a professional

  • Airbag (SRS) circuits must only be serviced by a professional. Never probe airbag connectors — accidental deployment causes serious injury.

At a glance

Severity: 4/5 – High4/5

Severity: High

Meaning
Driver Knee Bolster Deployment Control
System
Body Systems · Body
Safe to drive?
With caution
Typical repair cost
$100–$1,300
DIY difficulty
Professional only

Many vehicles have a knee airbag in the lower dash panel under the steering column. In a frontal crash it inflates against the driver's shins and knees, keeping the legs from hitting hard trim and helping hold the body in position for the main airbag. B0004 means the airbag control module found the knee airbag's firing loop out of range: open, shorted, or with resistance too high or low. Unlike the steering wheel airbag, the knee airbag does not pass through the clockspring; its harness runs along the column bracket and lower dash, an area people often disturb when they reach the fuse box, the OBD port or the brake pedal switch. While the fault is active the knee airbag can be switched off, and depending on the maker's strategy the rest of the frontal system may also be affected.

How to read B0004

How to read B0004B10203044
  1. B Body
  2. 0 Generic (SAE)
  3. 0 Body Systems
  4. 04 Specific fault

Symptoms

  • Airbag warning light on after startup
  • Light appeared after work under the dash: fuse box, OBD dongle, remote starter, dashcam or brake switch
  • Lower dash panel under the steering wheel loose, cracked or not clipped in
  • Warning after water got into the driver footwell (leaking windshield, cowl or door seal)
  • No change in how the car drives

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. Knee airbag connector pulled or not latched after work behind the lower dash panel 34%
  2. Harness chafed or pinched at the steering column bracket or panel clips 20%
  3. Corroded connector from water in the driver footwell 14%
  4. Knee airbag inflator out of range 13%
  5. Aftermarket installation (remote start, alarm, tracker) that tapped or damaged SRS wiring 11%
  6. Airbag control module fault 8%

Diagnosis step by step

Diagnostic flow: B00041Retrieve the code and its suffix2Think back to recent under-dash work3Visual check without touching theairbag4Check the footwell for water5Professional loop test6Clear and confirm

  1. Retrieve the code and its suffix

    An SRS-capable scan tool shows B0004 with a subtype such as open circuit or low resistance. An open circuit right after under-dash work almost always points to an unplugged or half-latched connector.

    Tools: OBD-II scanner

  2. Think back to recent under-dash work

    Knee airbags sit right where installers and DIYers work: fuse box, OBD port, brake pedal switch, steering column covers. Tell the shop exactly what was done, because it often saves an hour of diagnosis.

  3. Visual check without touching the airbag

    With a flashlight, look whether the lower dash panel is fully clipped in and whether any aftermarket wires run near the yellow SRS harness or its plugs. Don't pull the panel off and don't unplug anything yellow.

    Tools: Flashlight

  4. Check the footwell for water

    A wet carpet or musty smell under the driver's side points to a leak that can corrode the knee airbag connector. The leak has to be fixed as well, or the code will return.

  5. Professional loop test

    After disconnecting the battery and waiting the specified time, the technician swaps in an airbag simulator at the knee airbag plug. If the code clears, the knee airbag is replaced; if not, the harness back to the module is repaired with approved SRS connectors and splices.

    Tools: OBD-II scanner

  6. Clear and confirm

    After the repair the code is cleared with the scan tool and the ignition cycled with nobody in the car during reconnection. The SRS light must go out after its self-check.

    Tools: OBD-II scanner

Repair options & costs

RepairDIY?Estimated cost
SRS diagnosis No $100–$200
Reconnect or repair knee airbag connector/harness No $100–$350
Remove faulty aftermarket wiring and repair SRS harness No $150–$450
Replace driver knee airbag No $450–$1,200
Replace airbag control module No $500–$1,300

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Emissions / smog test impact

Depends

SRS faults don't affect the emissions part of a US smog check. States with a periodic safety inspection may fail a car with the airbag warning light on.

Frequently asked questions

What is a knee bolster airbag?

It's an airbag hidden in the lower dash panel under the steering wheel. In a frontal crash it cushions the driver's knees and shins and helps keep the body in a good position for the main airbag.

The light came on after I plugged in an OBD dongle. Is that related?

Possibly. The OBD port and the knee airbag are close together, and reaching behind the panel can tug the airbag connector. The dongle itself doesn't cause B0004, but the disturbance while fitting it can.

Is the knee airbag connected through the clockspring?

No. Only the steering wheel airbag and buttons use the clockspring. That's why a clockspring replacement usually doesn't fix B0004, unless the knee airbag plug was disturbed during that job.

Can I push the connector back in myself?

Don't. Working near a live airbag without disconnecting the battery and waiting out the reserve energy can fire it into your legs. A shop does this safely and clears the code afterwards.

Sources