OBDII DTC P0651 indicates that the vehicle’s computer has detected an open circuit on a particular sensor which is designated “B”. When the circuit is open, it may also indicate that it is absent. This sensor is associated with the automatic transmission or a differential. It is usually accompanied by another specific sensor code. This code indicates is that the circuit is open.
The reference voltage, which is usually 5 volts, is applied to the sensor via a switched circuit, plus a ground signal. Its resistance may be variable depending on the conditions to complete a circuit and provide the Powertrain Control Module (PCM) with an input voltage signal. If the PCM does not receive such a voltage signal, it is considered an open circuit.