Body, generic

B0004 Driver Knee Bolster Deployment Control

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

A knee airbag, also called an active knee bolster, sits in the lower dash below the steering column. It fills the space in front of the driver's knees and holds the lower body back in a frontal crash, mainly for unbelted occupants. B0004 reports a fault in its firing loop. Mercedes-Benz stores B000413 for an open squib of the driver kneebag. Ford lists B0004:1A with the passenger knee code B0013 on the 2024 Ranger and traces it to certain airbag connectors, recognisable by a number moulded into the connector lock (TSB 24-2444). In an IIHS study of more than 400 crash tests, knee airbags brought only a small benefit for belted dummies. Still, the restraint module counts the loop as part of the system, and the airbag lamp stays on until the circuit is repaired.

Likely causes

Faulty or loose knee airbag connector under the dash (Ford TSB 24-2444)
Harness pinched or damaged during work below the steering column
Internal fault in the knee airbag module

Symptoms

  • Airbag warning lamp on
  • Driver knee airbag may not deploy in a frontal crash
  • Safety note: do not probe the airbag circuit yourself; unplug airbag connectors only with the battery disconnected and the stated wait time observed

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€100–900
Time1–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

knee airbag modulesrs wiringsrs control module

Related codes

B0013B0001B0014

References: static.nhtsa.gov, static.nhtsa.gov, iihs.org, autoliv.com.