This sensor monitors the rotational speed of the turbocharger shaft and sends the signal to the engine control unit. It is mounted in the turbocharger housing, where it reads the passing of the compressor wheel or turbine blades. A failed or intermittent signal causes the ECU to enter a derated power mode or trigger a fault code, as the system can no longer verify boost pressure delivery. The sensor uses a magnetic or Hall-effect pickup, which requires a clean air gap between the sensor tip and the rotating target. On reinstallation, ensure the sensor is seated fully and the retaining bolt is torqued to specification to prevent vibration-induced signal dropout.