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P0641 Severity
Once the vehicle's onboard system no longer detects the issue, most codes will disappear on their own. If your vehicle still has this code, however, it should be properly diagnosed to avoid a more serious problem.
For a proper diagnosis, RepairPal Certified shops have the equipment, tools, and expertise.
Moderate severity codes might not threaten the vehicle's safety or functionality immediately, but you should still get it diagnosed within a reasonable time frame to keep your car running well.
The engine control module (PCM) sends out a 5 volt reference signal to all applicable engine sensors. If the PCM detects an incorrect output signal, it will set code P0641.
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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.
Get it diagnosed by a professional
Code P0641 is typically caused by one of the following:
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.
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.
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.
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.
Code P0641 is set when the engine is running and the PCM detects a voltage out of tolerance condition for more than 2 seconds.