P0651 Open circuit of the reference voltage B sensor

OBD-II Trouble Code - P0651 - Data Sheet

P0651 - Open circuit of the reference voltage of the sensor "B"

What does trouble code P0651 mean?

This Diagnostic Trouble Code (DTC) is a generic transmission code, which means it applies to OBD-II equipped vehicles. Although general, specific repair steps may differ depending on the brand / model.

When I find a stored code P0651, it means the powertrain control module (PCM) has detected an open circuit for a specific sensor; denoted in this case as "B". When diagnosing an OBD-II code, the term "open" can be replaced with "missing".

The sensor in question is usually associated with an automatic transmission, transfer case, or one of the differentials. This code is almost always followed by a more specific sensor code. P0651 adds that the circuit is open. Consult a trusted source of vehicle information (All Data DIY is a great choice) to determine the location (and function) of the sensor related to the vehicle in question. If P0651 is stored separately, suspect a PCM programming error has occurred. Obviously, you will need to diagnose and repair any other sensor codes before diagnosing and repairing P0651, but be aware of the open "B" circuit.

A voltage reference (typically five volts) is applied to the sensor in question through a switchable (key-powered) circuit. There should also be a ground signal. The sensor is likely to have a variable resistance or electromagnetic variety and is closing a specific circuit. The resistance of the sensor decreases with increasing pressure, temperature or speed and vice versa. Since the resistance of the sensor changes with conditions, it supplies the PCM with an input voltage signal. If this voltage input signal is not received by the PCM, the circuit is considered open and P0651 will be stored.

The Malfunction Indicator Lamp (MIL) may also be illuminated, but be aware that some vehicles will take multiple driving cycles (with a malfunction) for the MIL to turn on. For this reason, you must allow the PCM to enter standby mode before assuming that any repair is successful. Just remove the code after repair and drive as normal. If the PCM goes into readiness mode, the repair was successful. If the code is cleared, the PCM will not go into ready mode and you will know that the problem still exists.

Severity and symptoms

The severity of a stored P0651 depends on which sensor circuit is in the open state. Before you can determine the severity, you need to review the other stored codes.

The symptoms of a P0651 code can vary, so it's important to understand the various issues you may be experiencing. This may include difficult or no starting, the engine may begin to run rough, or you may notice that the vehicle no longer has the same fuel efficiency as it used to. The engine may misfire and you will likely notice a lack of power in the car. If a DTC is stored and the Check Engine light is off, the code may show as pending. However, in other cases, the engine light may be on.

Symptoms of a P0651 code may include:

• Inability to switch the transmission between sport and economy modes
• Gear shift malfunctions
• Delay (or lack) of turning on the transmission
• Transmission failure to switch between XNUMXWD and XNUMXWD
• Failure of the transfer case to switch from low to high gear
• Lack of inclusion of the front differential
• Lack of engagement of the front hub
• Incorrect or not working speedometer / odometer

Causes of the P0651 code

Possible causes of this engine code include:

• Open circuit and / or connectors
• Defective or blown fuses and / or fuses
• Faulty system power relay
• Bad sensor
• Defective engine control module (EMC).
• Open or short in ECM harness.
• Poor electrical connection quality in ECM circuits
• A shortened sensor on a 5 volt circuit.

Diagnostic and repair procedures

To diagnose a stored P0651 code, I will need access to a diagnostic scanner, a digital volt / ohmmeter (DVOM), and a trusted source of vehicle information (such as All Data DIY). A handheld oscilloscope can also be useful under certain circumstances.

Use your vehicle's information source to determine the location and function of the sensor in question as it relates to your specific vehicle. Check system fuses and full load fuses. Fuses that may appear normal when the circuit is very lightly loaded, often fail when the circuit is fully loaded. Blown fuses should be replaced, keeping in mind that a short circuit is likely the cause of the blown fuse.

Visually inspect the harness and connectors associated with the sensor system. Repair or replace damaged or burnt wiring, connectors, and components as needed.

Then I connected the scanner to the car diagnostic socket and got all stored DTCs. I like to write them down along with any associated freeze frame data, as this information can be helpful if the code turns out to be choppy. After that, I would go ahead and clear the code and test drive the car to see if it resets immediately.

If all system fuses are OK and the code resets immediately, use the DVOM to test the reference voltage and ground signals on the sensor in question. Generally, you should expect to have five volts and a common ground at the sensor connector.

If voltage and ground signals are present at the sensor connector, continue testing the sensor resistance and integrity levels. Use your vehicle information source to get test specifications and compare your actual results with them. Sensors that do not meet these specifications should be replaced.

Disconnect all related controllers from the system before testing resistance with the DVOM. If there is no voltage reference signal at the sensor, disconnect all associated controllers and use the DVOM to test the circuit resistance and continuity between the sensor and the PCM. Replace open or shorted circuits as necessary. If using a reciprocating electromagnetic sensor, use an oscilloscope to track the data in real time; paying particular attention to glitches and fully open circuits.

Additional diagnostic notes:

• This type of code is usually provided as support for more specific code.
• A stored code P0651 is usually associated with the transmission.

Common Mistakes When Diagnosing Code P0651

The biggest mistakes when diagnosing a P0651 code, or any other diagnosis for that matter, is to waste time looking at the symptoms and stored codes present in the vehicle, which may have nothing to do with the P0651 code. If the mechanic tries to rush the job or skips part of the testing process, they won't get an accurate result and will often misdiagnose the actual vehicle problems. Naturally, this will lead to a repair that will not work.

How serious is the P0651 code?

The P0651 sensor code can be a big problem for your vehicle. Although the vehicle can still be driven, at least for a time, fuel efficiency may suffer. As mentioned earlier, in some cases the car will be harder to start and may not start at all. It's always a good idea to get any problems with your car fixed as quickly as possible.

What repairs can fix code P0651?

The following are some of the common ways to troubleshoot code P0651:

• Have a mechanic use an OBD-II scanner to check the code.
• Reset the codes, after which the mechanic will take the car for a road test. In many cases the code is not returned.
• Check connectors, circuits and wiring - replace if necessary .
• Check the engine control module and all components of this circuit.

Additional comments to consider regarding code P0651

Many vehicles start to have sensor problems as they get older and drive more miles, and this doesn't always mean it's a sign of concern. Have a mechanic check with an OBD-II scanner, test the vehicle and reset the codes. In many cases, the codes are not returned and you will not need to have your vehicle repaired. However, if the code comes back and your car's problems continue or get worse, you'll want to take it in for repair.

What is P0651 Engine Code [Quick Guide]

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