B0012 is set when the airbag control module detects a fault in the passenger‑front airbag stage‑3 deployment circuit. Stage‑3 is one of the multiple inflator stages that determine the force with which the passenger airbag is deployed; a malfunction in the wiring, connector, or squib will illuminate the SRS light and may prevent that stage from firing in a crash.
Open, short, or high‑resistance paths in the harness or at the module connector are the most common source of B0012. Cut wires, broken insulation, or corrosion from moisture can interrupt the stage‑3 control signal.
The internal squib (igniter) for stage‑3 can fail internally, presenting as an open or abnormal resistance that the module recognizes as a fault.
A failed fuse or one that is intermittently opening can cause a loss of power or ground to the stage‑3 circuit, triggering the code.
In some vehicles the passenger‑airbag circuit passes through the clock‑spring; wear, breakage, or poor contact there can generate B0012.
Insufficient voltage or a bad ground at the SRS module can prevent proper circuit monitoring, leading to false fault detection.
Rarely the module itself may misinterpret the stage‑3 circuit; this is typically diagnosed after all harness and component tests pass.
The definition is universal; the diagnostics are engine-specific. Open your vehicle for the codes documented for it:
Low battery voltage can affect SRS module operation, but the code is normally triggered by an actual fault in the stage‑3 circuit, not simply by low voltage.
No. The PCM (engine control module) has no role in the airbag circuit; the SRS/airbag control module is a separate unit, and replacing the PCM rarely fixes B0012.
The light indicates a known fault in the supplemental restraint system. The passenger airbag may not deploy correctly in a crash, so the vehicle should be repaired promptly.
Yes. Impact damage to wiring, connectors, or the airbag module can create an open or high‑resistance condition that sets the code, even if the airbag was not fired.