B0004 code meaning
Driver Knee Bolster Deployment Control
What B0004 means
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.
B0004 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyWorkshop only
- Typical labor1 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of B0004
- 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
Common causes of B0004
- Most likelyFaulty or loose knee airbag connector under the dash (Ford TSB 24-2444)
- PossibleHarness pinched or damaged during work below the steering column
- Less likelyInternal fault in the knee airbag module
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Knee airbag moduleSRS wiringSRS control module
What each part of B0004 means
| B | Body system: airbags, seat belts, power seats, lights and climate control. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 04 | The exact fault in that area. |
What to do next
- Note B0004 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about B0004
What does B0004 mean?
B0004 means "Driver Knee Bolster Deployment Control". A knee airbag, also called an active knee bolster, sits in the lower dash below the steering column. It is a generic code, so it means the same on every make.
Is it safe to drive with B0004?
B0004 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will B0004 fail an emissions test?
B0004 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is B0004 to fix?
Difficulty: workshop only. A typical repair takes 1 to 3 hours of work. This is a job for a workshop.
Will B0004 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.