|Trouble Code||Fault Location||Probable Cause|
|P0651||Sensor reference voltage B -circuit open||Wiring short to positive|
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What Does Code P0651 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.
What are the common causes of code P0651 ?
The possible causes of open circuits are any and varied, but some common causes could include the following-
- Damaged, shorted, burnt, disconnected, and corroded wiring and/or connectors
- One or more defective sensors that are connected to the “B” reference voltage circuit
- Blown fuses, or defective relays on some applications
- Failed or failing PCM. Note that this is a rare event and the fault must be sought elsewhere before any controller is replaced.
What are the symptoms of code P0651 ?
Apart from a stored trouble code and an illuminated warning light, symptoms of this code are largely make-and-model specific on the one hand, and dependent on which sensors are affected by the failure of the “B” reference voltage circuit, on the other. Refer to the manual for the application being worked on for detailed information on the symptoms that are most likely to be present on the application when P0651 is present.
Note however, that while other codes may be present along with P0651, it must be remembered that these codes and their associated symptoms, which could include serious driveability issues, are almost certainly the result of P0651, which means that P0651 MUST be resolved first.
How do you troubleshoot code P0651 ?
NOTE: Apart from a repair manual for the application and a good quality digital multimeter, a color-coded wiring diagram, and reference data for all the sensors that use the “B” reference voltage circuit will be most helpful in diagnosing this code.
Record all fault codes present, as well as all freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.
NOTE: On some applications, a short circuit to battery voltage in the signal circuit of some sensors can cause P0651 to set as well. Thus, if codes relating to short circuits to voltage are present, consult the manual on the relationship between such codes and P0651 before starting a diagnostic procedure to save time, and to prevent possible misdiagnoses.
Refer to the manual to identify all the sensors that are connected to the “B” reference voltage circuit. Also, use this opportunity to determine to routing, color-coding, function, and location of all relevant wiring before unplugging, disconnecting, or testing anything.
It is also a good idea to trace and mark all relevant wiring on the wiring diagram at this point. Doing this now will save time and possible confusion later when individual wires have to be traced while looking for the cause of the open circuit.
Once all components and wiring are identified, refer to the manual on the correct procedure(s) to follow to determine that the PCM is actually generating a 5-volt current in the “B” reference voltage circuit. Typical values for this circuit range between 4.3 volts, to 4.7 volts, but be sure to consult the manual for the correct values for the application being worked on.
Be aware though that testing procedures vary between applications, so be absolutely sure to follow the directions in the manual EXACTLY to avoid damaging the PCM, or causing damage to the electrical system where there was none before.
ONLY proceed with the diagnostic procedure when it is certain that the PCM is delivering the correct voltage to the “B” reference voltage circuit. If it does not deliver the correct voltage, replace the PCM, and perform all reprogramming and/or relearning procedures prescribed in the manual to ensure that the replacement PCM works as intended.
If however, the PCM does deliver the correct current, attempt to clear all codes, but do not disconnect the scanner from the data link connector because you need to be able to monitor any changes in the status of the affected reference voltage circuit in real time.
Note that on some applications, it may not be possible to clear the code(s) until the problem is resolved. Nonetheless, proceed with the diagnostic procedure by disconnecting the wiring from each of the affected sensors one by one, while monitoring the scanner.
With some luck, the reading will change from indicating a “fault condition”, to displaying a reading that indicates the circuit to be OK, when the wiring that contains the open circuit is disconnected from the system. Note that the actual wording of the reading depends on the scanner.
If the reading does not change after all affected sensors have been disconnected, suspect an open circuit somewhere between the PCM and any of the affected sensors. However, before you open any harness (es), test the continuity of the reference circuit of each affected sensor between the connector and a suitable ground.
Typical values are usually between about 4.8 volts, and about 5.2 volts, but be sure to determine the exact value for each sensor by referring to the manual. Compare all obtained readings with the values stated in the manual, and make repairs to wiring as required to ensure that all reference voltage values fall within the manufacturer’s specifications.
NOTE #1: Note that a wire does not have to be physically broken or disconnected for an open circuit to be present. Some manufacturers, most notably Ford, is not known for the quality of the slip joints in its wiring, so be sure to check for continuity across all slip joints, soldered joints, or wire terminations when searching for an open circuit.
NOTE #2: Be aware that many other codes will set during this step, which is to be expected if sensors are disconnected. While most of these codes can be cleared after repairs are complete, note that some may require the completion of one or more drive or start cycles before they can be cleared. Consult the manual on this important point if some codes prove difficult or impossible to clear manually.
If all reference circuits check out, consult the manual on the correct procedure to follow to test the internal resistance of ALL affected sensors, since a failed sensor can also produce an open circuit. Compare all obtained resistance readings with the values stated in the manual, and replace all and any sensors that do not conform to the manufacturer’s specifications.
NOTE: Resist the temptation to replace sensors with aftermarket units, and especially MAF sensors on BMW, Mercedes-Benz, VW, and Audi applications. These applications are known not to tolerate aftermarket parts well, so saving a few dollars now will almost certainly cause a recurrence of the problem in the not-too-distant future.
The steps up to and including Step 6 will resolve code P0651 in nine out of every ten instances, but it is possible for an intermittent fault to cause sporadic failures in the “B” reference voltage circuit, as in any other circuit.
If an intermittent fault is suspected, it might be possible to trace and repair the problem, but non-professional mechanics should take note that this process involves testing and retesting every wire, every sensor, every connection, every wire termination, and every connector in the “B” reference voltage circuit, with no guarantee that the fault will ever be found.
In these cases, the wiser option is to replace the relevant harness (es) with new OEM wiring. While this might appear to be “throwing parts at the problem and hoping something will stick”, the reality is that with this type of problem simply replacing all relevant wiring and components is often the most cost effective (and quickest), way to solve the problem.
Clear all codes after repairs are complete, and operate the vehicle for at least one complete drive cycle before scanning the system again to see if P0651 (or any other code(s)) return. Refer to Step 7 above if any codes do return: if no codes return after completing several drive cycles , the repair can be considered as successful.
Codes Related to P0651
- P0652 – Relates to “Sensor Reference Voltage “B” Circuit Low”
- P0653 – Relates to “Sensor Reference Voltage “B” Circuit High”