P0651 Sensor Reference Voltage B Circuit Open
OBD-II Trouble Code Technical Description
Sensor Reference Voltage "B" Circuit Open
What does that mean?
This diagnostic trouble code (DTC) is a generic powertrain code, which means that it applies to OBD-II equipped vehicles. Although generic, the specific repair steps may vary depending on make/model.
When I find a stored P0651 code, it means that the powertrain control module (PCM) has detected an open circuit for a particular sensor; designated in this case as "B". When diagnosing an OBD-II code, the term open could be substituted for absent.
The sensor in question is typically associated with the automatic transmission, transfer case, or one of the differentials. This code will almost always be accompanied by a more specific sensor code. The P0651 adds that the circuit is open. Consult a reliable vehicle information source (All Data DIY is a great choice) to determine the sensor location (and function) as it pertains to the vehicle in question. If the P0651 is stored alone, suspect that a PCM programming error has occurred. Obviously, you will need to diagnose and repair any other sensor codes prior to diagnosing and repairing the P0651 - but keep the open "B" circuit in mind.
Reference voltage (typically five-volts) is applied to the sensor in question via a switched (energized with the key on) circuit. There should also be a ground signal. The sensor is probably of either the variable resistance or electromagnetic variety and completes a particular circuit. Sensor resistance decreases as pressure, temperature, or speed is increased and vice versa. As the sensor resistance changes with conditions it provides the PCM with an input voltage signal. If this input voltage signal is not received by the PCM, the circuit is considered open and a P0651 will be stored.
A malfunction indicator lamp (MIL) may also be illuminated but keep in mind that some vehicles will require multiple drive cycles (with a failure) for a MIL to be illuminated. For this reason, you should allow the PCM to enter readiness mode before considering any repair successful. Simply clear the code, after repairs are performed, and drive the vehicle normally. If the PCM enters readiness mode, the repair was successful. If the code is reset, the PCM will not enter readiness mode and you will know that you still have a problem.
Severity & Symptoms
The severity of a stored P0651 depends upon which sensor circuit is experiencing an open condition. One must consider other stored codes before a determination of severity can be made.
Symptoms of a P0651 code may include:
Failure of the transmission to shift between sport and economy modes Transmission shifting malfunctions Delayed (or no) transmission engagement Failure of the transmission to switch between all-wheel and two-wheel drive modes Failure of the transfer case to shift from low to high gear Lack of front differential engagement Lack of front hub engagement Erratic or inoperative speedometer/odometerCauses
Possible causes for this engine code include:
Open circuits and/or connectors Defective or blown fuses and/or fusible links Faulty system power relay Bad sensorDiagnostic and Repair Procedures
To diagnose a stored code P0651, I would need access to a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a trustworthy vehicle information source (like All Data DIY). A portable oscilloscope may also be helpful under certain circumstances.
Use your vehicle information source to determine the location and function of the sensor in question, as it relates to your particular vehicle. Check system fuses and fusible links with the circuit under a full load. Fuses that may appear normal when there is very little load on the circuit often fail when the circuit is
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