B0004: Driver Knee Bolster Deployment Control
What to do
Get this looked at promptly, and do not attempt it yourself. An airbag fault usually means the whole restraint system has switched itself off, so the airbags and pretensioners may not fire in a crash — the car drives normally and is far less able to protect you. Never probe airbag wiring with a multimeter: the current an ordinary meter uses can set off the charge. This needs a workshop with the right equipment and the correct disable procedure.
What this code means
The deployment loop for the driver knee bolster airbag is faulty.
A knee bolster airbag exists to stop a particular kind of injury that the frontal airbag cannot prevent: submarining.
In a frontal impact an occupant's hips try to slide forward under the lap belt. If they do, the belt rides up onto the abdomen — where it can cause severe internal injury — and the upper body rotates down, out of the path of the airbag that was aimed at it. The whole restraint geometry fails at once.
A knee bolster stops the pelvis moving forward, which keeps the lap belt on the hip bones where it belongs and keeps the torso where the frontal airbag expects to meet it. So it is not a comfort item or a minor supplement — it is what makes the main airbag work as designed.
It is also why it deploys downward and low, and why people who have seen one deploy often do not realise an airbag was involved.
Likely causes
Start here
- Corroded or loose connector · specialist
- Open circuit in the airbag deployment loop · specialist
- Faulty airbag module or initiator · specialist
Less common
- Faulty restraints control module · specialist
- Restraint device already deployed and not replaced · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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