Error Code P0651 is defined as Sensor Reference Voltage B Circuit Open. This means a fault has been detected in sensor reference voltage “B”, a problem likely caused by a faulty electrical connection or a problem in the engine control module.

This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to present. Specifications on the definition, troubleshooting, and repairs, of course, vary from make, model, and power configuration.

When Error Code P0651 is stored, this means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) has detected an open circuit for a specific sensor; which in this case is designated as “B”. In diagnosing for this code, the term ‘open’ cold be substituted for ‘absent’.

Usually, the sensor in question is associated with the automatic transmission, one of the differentials, or transfer case. This code also almost always comes with a more specific sensor code. The P0651 means the circuit is open. Thus, make sure you consult with your reliable vehicle source information to determine the location (and function) of that sensor. If this code is stored, this means the PCM programming error has occurred. This, of course, requires you to diagnose and repair other sensor codes prior to diagnosing and repairing Error Code P0651 – keep the open “B” circuit in mind.

The reference voltage (usually 5V) is applied to the sensor in question through a switched (energized with key on) circuit. Also, there must be a ground signal. The sensor is maybe of either the variable resistance or electromagnetic variety and completes a specific circuit. Sensor resistance goes down as pressure, temperature, or vehicle speed increases, and vice versa. As the sensor resistance changes with the conditions, it provides the PCM with an input voltage signal. If the PCM does not receive this input voltage signal, then the circuit is considered open, and this code will be set.

For some vehicles, it may require multiple drive cycles before the Check Engine light is activated. For this, you must allow the PCM to enter readiness mode before addressing the problem.

Thus, simply clear the code after repairs are made, and then drive the vehicle normally. If the PCM enters readiness mode, then that means the repair was successful.

If the code resets, on the other hand, this means the PCM is unable to enter readiness mode, and you still have to proceed on your diagnosis.

Common Symptoms

Symptoms of this code vary on its severity, particularly on the extent of the open condition. Some of this code’s common symptoms include:

  • Malfunction of transmission shifting
  • Failure of transmission to shift from sport to economy mode
  • Delayed or no transmission engagement
  • Failure of the transfer case to shift from low to high gear
  • Failure of the transmission to switch between all-wheel and two-wheel drive modes
  • Lack of front hub engagement
  • Lack of front differential engagement
  • Inoperative or erratic speedometer/odometer

Possible Causes

There are many causes of this code, such as:

  • Open in the circuits and connectors
  • Failed system power relay
  • Blown or defective fuses and fusible links
  • Defective sensor

How to Check

As with the majority of other error codes, diagnosing this code require a diagnostic scanner, DVOM (digital volt/ohmmeter), and a dependable vehicle source information. A portable oscilloscope can be of help under certain circumstances.

Refer to the vehicle information source to locate and understand the function of the sensor in question. Check the system fuses and fusible link with the circuit under a full load. Fuses that may seem normal when there is a very small load on the circuit often fail when the circuit is fully loaded. Any blown fuses must be replaced. Remember that shorted circuits likely cause a blown fuse.

Check the sensor system related to the connectors and wiring harness. Repair or replace any damaged and burnt connectors and components.

Next, connect your diagnostic scanner to the vehicle’s diagnostic connector, get all stored trouble codes too. You can write these codes down, including their freeze frame data, as this information can be of good help for you. Then, clear the code and then take the vehicle for a test drive to see if the vehicle comes back.

If all fuses in the system look to be in good shape, and the code immediately resets, then check for test reference voltage and a ground signal (again, using the DVOM). Usually, you can expect a 5V and a common ground at the sensor connector.

If both voltage and ground signal are present, then continue your diagnosis by testing the sensor resistance and continuity levels. Again, you can refer to the vehicle information source for the specific info.

Next, unplug al related controllers from the system circuits before testing resistance with the DVOM. If there are no reference voltage signal present at the sensor, then disconnect all related controllers and use the DVOM to check for circuit resistance and continuity between the PCM and the sensor.

If the vehicle uses an electromagnetic sensor that comes with a reciprocating signal, then you can use the oscilloscope to monitor live data. Make sure you pay close attention to the glitches and circuits that are completely open.

How to Fix

  • Replacement of blown fuses
  • Repair or replacement of damaged or burnt components or connectors
  • Repair or replacement of open or shorted circuits

Error Code P0651 usually comes as a support for a more specific error code. Also, this fault is usually associated with the drivetrain.