P0651 – Sensor Reference Voltage B Circuit/Open

What is the P0651 Code?

The P0651 is a generic diagnostic trouble code (DTC) that stands for "Sensor Reference Voltage 'B' Circuit Open". It indicates that the powertrain control module (PCM) has detected an open circuit condition in the reference voltage circuit designated as "B" for a particular sensor.

Urgency and Severity

When the P0651 code is stored, short distance driving is still permitted in most cases. However, it's recommended to get the vehicle diagnosed and repaired as soon as possible to prevent potential drivability issues or damage to components. The severity depends on which specific sensor circuit is affected.

Applicability

The P0651 is a generic code that applies to all vehicles equipped with OBD-II systems. However, the exact troubleshooting procedures, affected sensors, and repair steps may vary between different makes and models.

Associated Components

The sensor reference voltage "B" circuit provides a 5V reference signal from the PCM to various sensors, which commonly include:

• Throttle Position Sensor (TPS)
• Mass Air Flow (MAF) sensor
• Manifold Absolute Pressure (MAP) sensor
• Accelerator Pedal Position (APP) sensor
• Fuel Tank Pressure (FTP) sensor

These sensors use the reference voltage to produce output signals that help the PCM monitor and control engine functions.

Symptoms

Common symptoms of a P0651 code include:

• Illuminated check engine light
• Engine may be hard to start or not start at all
• Rough running or misfiring
• Reduced fuel efficiency
• Lack of power under acceleration
• Transmission shifting issues

Causes

Potential causes for a P0651 code are:

• Faulty wiring or connectors in the 5V reference circuit
• Short to ground in the sensor signal or reference wires
• Open circuit in the sensor ground connection
• Blown fuse for the 5V reference voltage supply
• Defective sensor (e.g. TPS, MAF, MAP, APP)
• Malfunctioning PCM (rare)

Diagnosis

To properly diagnose a P0651 code, follow these steps:

1. Connect a scan tool and retrieve all stored trouble codes and freeze frame data.

2. Visually inspect the wiring harness and connectors for the 5V reference "B" circuit. Look for damaged, loose, or corroded wires and connectors. Repair as needed.

3. Check the voltage at the 5V reference wire for each sensor using a digital multimeter (DMM). It should be approximately 5V with the key on. If the voltage is low or missing, suspect an open or short to ground.

4. Test the resistance and continuity of the 5V reference and sensor ground wires between the PCM and each sensor. Repair open or shorted wires.

5. If the reference voltage and wiring check out okay, test each sensor independently to verify proper operation per manufacturer specs. Replace faulty sensors.

6. If all sensors and wiring are good, the PCM may be suspect, although this is rare. Consult a professional technician for PCM diagnosis and replacement.

Repair Costs

The cost to diagnose and repair a P0651 code can vary depending on the root cause and labor rates. Typical costs are:

• Diagnosis: $75-150 USD ($100-200 CAD/EUR)
• Wiring repairs: $50-200 USD ($70-270 CAD/EUR)
• Sensor replacement: $75-400 USD ($100-540 CAD/EUR)
• PCM replacement: $500-1500 USD ($675-2000 CAD/EUR)

Prevention

To help prevent P0651 code issues:

• Maintain the engine wiring harness and keep it away from heat sources
• Avoid spilling fluids on electrical connectors
• Address any sensor-related codes promptly
• Perform regular vehicle maintenance per manufacturer schedule

Associated Codes

Other codes commonly diagnosed along with P0651 include:

• P0100-P0104 (MAF sensor circuit)
• P0105-P0109 (MAP sensor circuit)
• P0120-P0124 (TPS circuit)
• P2135 (TPS/APP correlation)

Frequently Asked Questions about P0651 Code

What does the P0651 code mean?

The P0651 is a generic diagnostic trouble code (DTC) that stands for “Sensor Reference Voltage ‘B’ Circuit Open”. It indicates that the powertrain control module (PCM) has detected an open circuit condition in the reference voltage circuit designated as “B” for a particular sensor.

Is it safe to drive with a P0651 code?

When the P0651 code is stored, short distance driving is still permitted in most cases. However, it’s recommended to get the vehicle diagnosed and repaired as soon as possible to prevent potential drivability issues or damage to components.

What vehicles can get the P0651 code?

The P0651 is a generic code that applies to all vehicles equipped with OBD-II systems. However, the exact troubleshooting procedures, affected sensors, and repair steps may vary between different makes and models.

Which sensors commonly use the 5V reference voltage circuit?

The sensors that commonly use the 5V reference voltage “B” circuit include:

• Throttle Position Sensor (TPS)
• Mass Air Flow (MAF) sensor
• Manifold Absolute Pressure (MAP) sensor
• Accelerator Pedal Position (APP) sensor
• Fuel Tank Pressure (FTP) sensor

What are the common symptoms of a P0651 code?

Common symptoms include:

• Illuminated check engine light
• Hard starting or no start condition
• Rough running or misfiring
• Reduced fuel efficiency
• Lack of power under acceleration
• Transmission shifting issues

What are the potential causes of a P0651 code?

Potential causes include:

• Faulty wiring or connectors in the 5V reference circuit
• Short to ground in sensor signal or reference wires
• Open in the sensor ground connection
• Blown fuse for the 5V reference voltage
• Defective sensor (e.g., TPS, MAF, MAP, APP)
• Malfunctioning PCM (rare)

How do you diagnose a P0651 code?

To diagnose:

1. Scan for codes and document freeze frame data
2. Visually inspect wiring and connectors
3. Check reference voltage and ground at each sensor
4. Test wiring continuity and resistance
5. Test sensors and replace if faulty
6. Suspect PCM if all wiring and sensors check out okay

What tools are needed to diagnose a P0651 code?

To properly diagnose a P0651 code, you will need:

• An OBD-II scanner to retrieve codes and data
• A digital multimeter (DMM) to check voltages, resistance, and continuity
• A wiring diagram for the specific vehicle
• The manufacturer’s diagnostic and repair information

How much does it cost to diagnose and fix a P0651 code?

The cost to diagnose and repair a P0651 code can vary depending on the root cause and labor rates. Typical costs are:

• Diagnosis: $75-150 USD
• Wiring repairs: $50-200 USD
• Sensor replacement: $75-400 USD
• PCM replacement: $500-1500 USD

Can a blown fuse cause a P0651 code?

Yes, a blown fuse that supplies the 5V reference voltage to the sensors can cause a P0651 code to be set. The fuse should be checked early in the diagnostic process.

Can a P0651 code cause a no-start condition?

Yes, in some cases a P0651 code can cause a no-start condition if the affected sensor is critical for starting, such as the throttle position sensor or crankshaft position sensor.

Will a P0651 code clear itself?

No, a P0651 code will not clear itself even if the underlying problem is repaired. The code needs to be cleared with a scan tool after repairs are complete.

How do you test the 5V reference voltage at a sensor?

To test the 5V reference voltage at a sensor:

1. Locate the sensor and disconnect its electrical connector
2. Turn the key on (engine off)
3. Using a multimeter set to DC volts, probe the reference voltage pin in the connector
4. The reading should be approximately 5V, otherwise there is a problem with the reference circuit

What does an open circuit mean in relation to a P0651 code?

An “open circuit” in the context of a P0651 code refers to a break in the continuity of the 5V reference voltage circuit between the PCM and the sensor. This can be caused by a broken wire, poor connection, or faulty PCM.

Can a P0651 code affect transmission shifting?

Yes, a P0651 code can affect transmission shifting if the faulty sensor is related to the transmission, such as the throttle position sensor or vehicle speed sensor. Harsh shifts, delayed engagement, or no shifting can occur.

What other codes may be set along with a P0651?

Other codes that may accompany a P0651 include:

• P0100-P0104 (MAF sensor circuit)
• P0105-P0109 (MAP sensor circuit)
• P0120-P0124 (TPS circuit)
• P2135 (TPS/APP correlation)

Can a malfunctioning alternator cause a P0651 code?

Indirectly, yes. A malfunctioning alternator that is overcharging or undercharging can cause damage to sensitive electronic components over time, including sensors and the PCM. This could lead to a P0651 code in some cases.

How do you repair a short to ground in the 5V reference circuit?

To repair a short to ground in the 5V reference circuit:

1. Locate the shorted wire using a wiring diagram and testing with a multimeter
2. Disconnect the PCM and sensor connectors
3. Repair the damaged wire or replace the harness as needed
4. Reconnect the components and clear the code
5. Test drive to verify the repair

Can a P0651 code be caused by a bad ground connection?

Yes, a poor ground connection at the PCM, engine block, or chassis can cause voltage feedback in the 5V reference circuit, setting a P0651 code. Ground connections should be checked for looseness, corrosion, or damage.

How can you prevent a P0651 code from reoccurring?

To help prevent a P0651 code from reoccurring:

• Maintain the engine wiring harness and keep it away from heat sources
• Avoid spilling fluids on electrical connectors
• Address any sensor-related codes promptly
• Perform regular vehicle maintenance per the manufacturer’s schedule