What does it mean?
OBD II fault code P0651 is a generic code that is defined as “Sensor reference voltage “B” – circuit open”, and is set when the PCM (Powertrain Control Module) detects an open circuit in one of the reference voltage circuits it supplies to various engine and other sensors. Note that “B” circuit is sometimes referred to as “Circuit 2” In practical terms, reference voltages (usually 5 volts), can be seen as the power supply that some sensors use to generate a signal voltage.
For instance, when the PCM delivers a 5-volt reference voltage to say, the MAF (Mass Airflow) sensor, the current changes because the elements’ resistance increases, or decreases according to the volume of air that flows over the sensors’ heated element.
If the sensor elements’ resistance increases, less current is returned to the PCM via a dedicated signal circuit, with the difference between the original reference voltage value and the voltage value of the returned signal representing the volume of air that enters the engine at any given moment. The opposite happens when the sensor elements’ resistance decreases; more voltage is returned to the PCM, but the difference still represents the volume of air that enters the engine.
While the above example is an oversimplification, it serves to illustrate the point that the PCM uses reference voltages to monitor both the operation of a particular component (like a sensor), and the system that depends on the efficient operation of that sensor to function as intended. As a practical matter, the PCM generates two stabilized 5-volt reference voltage circuits, with each circuit serving different sensors.
Typically, reference voltage circuit #1 would serve the following sensors- MAP (Manifold Absolute Pressure) sensor FTP (Fuel Tank Pressure) sensor A/C (Air Conditioning) refrigerant pressure sensor APP (Accelerator Pedal Position) sensor #2 CMP (Intake Camshaft Position) sensor, – while reference voltage circuit #2 would serve the following sensors- APP (Accelerator Pedal Position) sensor #1 TP (Throttle Position) sensors #1 & #2 CK (crankshaft Position) sensor NOTE: There are differences between manufacturers on which sensors are served by which reference voltage circuit.
Always refer to the manual for the application being worked on for detailed information on which reference voltage circuit serves which sensors to avoid confusion, misdiagnoses, and the almost certain unnecessary replacement of parts and components. This “division of labor” so to speak, means that should one sensor or circuit fail, the application will either not be immobilized, or it will be able to enter a failsafe, or limp mode.
However, the disadvantage of using only two reference voltage circuits to serve multiple sensors involves the fact that a failure in one reference voltage circuit affects all the other sensors that are served by that failed reference voltage circuit. This also means that one or more signals returned to the PCM by one or more sensors may be incorrect, invalid, implausible, or otherwise out of accepted parameters, even though the affected signal circuits may be in perfect working condition.
Nonetheless, to ensure that all sensors are continuously supplied with a stable 5-volt reference voltage, the PCM monitors both reference voltage circuits continually, and when it detects an open circuit in reference voltage circuit “B” for longer than about 0.5 seconds, it will set code P0651 and illuminate a warning light. The image below shows damage caused to wiring that had been chafing against engine components, which is a common cause of open circuits.