U0172 indicates the Powertrain Control Module (PCM) has lost communication with the Restraints System Sensor C, which monitors the status of the vehicle's restraint (airbag) system. The PCM cannot receive the sensor’s signal, so it sets a communication fault.
Connector pins or wiring can become damaged, corroded, or loose, breaking the communication path between the sensor and PCM.
The sensor itself may have failed internally, preventing it from sending a valid signal to the PCM.
The PCM’s communication circuit for the restraint system can develop a fault, often due to water intrusion or internal component failure.
A fuse that supplies power to the restraint system sensor circuit may be open, cutting power to the sensor.
Excessive voltage transients or a poor ground can disrupt the sensor’s signal, causing intermittent communication loss.
Replacing the PCM is rarely the correct fix. The majority of U0172 cases are due to wiring, connector, fuse, or sensor failures. PCM replacement should only be considered after all other possibilities have been eliminated and a PCM communication port defect is proven.
When U0172 is active, the PCM disables the airbag system for safety, so the airbags will not deploy in a crash until the fault is resolved.
The vehicle can be driven, but the restraint system is compromised. It is advisable to have the problem repaired promptly, especially if you rely on airbags for safety.
Severe voltage irregularities from a failing battery or alternator can affect sensor communication, but they are a less common cause compared to direct wiring or sensor faults.