U0171 indicates that the Powertrain Control Module (PCM) has lost communication with the Restraints System Sensor B, which monitors the deployment status of the vehicle's safety restraint system.
The sensor itself has failed electrically or mechanically, preventing it from sending valid data to the PCM.
Wire chafing, corrosion, or broken pins in the connector to Sensor B can interrupt the communication line.
A connector that is not fully seated or has bent pins can cause intermittent loss of signal.
Only after ruling out sensor, wiring, and connector issues should a PCM internal fault be considered.
No. Most U0171 cases are caused by a faulty sensor, wiring, or connector. Replacing the PCM is rarely necessary and is not the recommended first step.
If the underlying problem (sensor, wiring, or connector) is not repaired, the code will return and the airbag system may remain disabled.
It can appear alongside other restraint system codes (e.g., U0170, U0100) if the communication loss affects multiple modules, but each code has its own specific source.
A standard OBD‑II scan tool for reading the code and a multimeter (or oscilloscope) for checking voltage and signal integrity are sufficient.