B0004 indicates a fault in the deployment control circuit for the driver knee bolster airbag. The SRS control module has detected an open, short-to-ground, short-to-power, or resistance problem in the circuit that fires the driver knee airbag inflator. This turns on the airbag warning light and disables that airbag until the fault is repaired.
The inflator squib itself can fail with an open coil, internal short, or out-of-spec resistance, making the SRS module report a deployment-control circuit fault.
Harnesses routed under the steering column and dash can chafe, get pinched, corrode, or suffer terminal push-out, causing opens or shorts in the deployment circuit.
A loose, corroded, or improperly seated connector at the knee bolster can add resistance or create an intermittent open that the SRS module reads as a deployment-circuit fault.
Corrosion, bent pins, or poor contact at the airbag control module connector can interrupt the driver knee bolster deployment circuit and set this code.
Although uncommon, the SRS control module can have an internal driver or sensing failure for the knee bolster circuit; it should only be considered after the squib and wiring are proven good.
The vehicle will drive normally, but the driver knee airbag may not deploy in a crash, so the SRS system should be diagnosed and repaired promptly.
No. The engine computer does not control the airbag system. This is an SRS circuit fault; the rare module cause is the SRS or airbag control module, not the PCM.
Clearing the code may reset the light temporarily, but the light and code will return if the underlying circuit fault is still present. Clearing alone is not a repair.
Yes. Diagnosis requires an SRS-compatible scan tool and a digital multimeter, plus strict safety procedures for depowering the airbag system before probing connectors.