P0641 OBD-II: Sensor Reference Voltage "A" Circuit/Open

What does code P0641 mean?

Code P0641 stands for Sensor Reference Voltage 'A' Circuit Open.

The vehicle's computer, the powertrain control module (PCM), supplies a reference voltage to many of the onboard sensors. Typically, this reference signal is 5-Volts. The sensors then verify this voltage signal in accordance with whatever they're measuring and send it back to the PCM.

Although the external sensor reference circuits are independent of one another, they are sometimes connected inside the PCM. For example, on some General Motors vehicles, the engine oil pressure sensor and manifold absolute pressure sensor are bussed together inside the PCM. This can cause one sensor's 5-Volt reference circuit to affect the other.

Code P0641 indicates the PCM has detected voltage is out of specification on the 5-Volt reference circuit. The 'A' part of the code indicates the problem is with the number one sensor, if more than one sensor is in question.

Note: Typically, the manufacture's repair information will indicate which sensor circuits are affected when code P0641 is stored. For example, on one vehicle, code P0641 may indicate a problem with the power take off (PTO) reference circuit. On another vehicle, it may refer to the fuel pressure sensor.

P0641 symptoms

• An illuminated check engine light
• Sensor-related performance problems

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Common causes for P0641

Code P0641 is typically caused by one of the following:

• A faulty sensor
• Wiring issues
• A problem with PCM

How to diagnose and repair P0641

Perform a preliminary inspection

Sometimes P0641 can pop up intermittently. This is especially true if the code is a history code and not current. Clear the code and see if it returns. If it does, the next step is to perform a visual inspection. A trained eye can check for issues such as broken wires and loose connections. If a problem is found, the issue should be repaired and the code cleared. If nothing is discovered, check for technical service bulletins (TSBs). TSBs are recommended diagnostic and repair procedures put out by the vehicle manufacturer. Finding a related TSB can greatly reduce diagnostic time.

Check the circuit

After the primary inspection the reference circuits should be checked. This can be done using a digital multimeter (DMM). A sensor typically has two or three wires going to it, one of which is the reference signal. The meter should show a good reference voltage being supplied to the sensor(s) by the PCM. If not, the wiring between the sensor and PCM should be checked and repaired as needed.

Check the sensors

Typically, the next thing a technician will do is check the sensor(s). This can be accomplished using a diagnostic scan tool connected to the vehicle diagnostic port. The data produced by the sensor in question can be compared the manufacture's specifications. If the value is out of specification, the sensor is probably faulty and should be replaced.

Sensor operation can also be checked directly at the sensor using a DMM or oscilloscope. Once again, the values viewed on either of these tools should coincide with the manufacturer's specifications.

Check the PCM

In rare cases, the PCM may be at fault. The PCM should supply a 5-volt reference to the sensors in question. If it does not, it may be faulty or require reprogramming.

Other diagnostic codes related to P0641

• P0642: Code P0642 indicates the powertrain control module (PCM) has detected a low reference voltage for the sensor 'A' circuit. This typically indicates a short circuit.
• P0643: Code P0643 indicates the powertrain control module (PCM) has detected a high reference voltage for the sensor 'A' circuit. This typically indicates an open circuit.

Code P0641 technical details

Code P0641 is set when the engine is running and the PCM detects a voltage out of tolerance condition for more than 2 seconds.