P0641 – Sensor reference voltage A -circuit open

By Reinier (Contact Me)
Last Updated 2017-06-01
Automobile Repair Shop Owner

Table of Contents

1. What Does Code P0641 Mean?
2. What are the common causes of code P0641 ?
3. What are the symptoms of code P0641 ?
4. How do you troubleshoot code P0641 ?
5. Codes Related to P0641
6. Get Help with P0641

What Does Code P0641 Mean?

OBD II fault code P0641 is a generic code that is defined as “Sensor reference voltage “A” -circuit open”, and is set when the PCM (Powertrain Control Module) detects an open circuit in the “A” reference voltage circuit. Most applications use two separate reference voltage circuits to supply a 5V current to engine and other sensors. In most cases, “Reference Voltage Circuit “A” refers to the circuit that supplies a voltage to sensors that control/monitor the drive train and transmission, while a separate circuit, referred to as Reference Voltage Circuit “B”, refers to the circuit that delivers a reference voltage to sensors that control/monitor engine and driveability functions. Note that with both circuits, several sensors and control modules are inter-connected via the CAN (Controller Area Network) bus system, which means that a failure of circuit “A” can affect all the sensors, control modules, and sub-systems that share that circuit.

NOTE: Not all sensors require a reference voltage to work: two-wire sensors are almost always Hall-effect sensors that generate their own signal currents by means of a magnetic field that is interrupted by a reluctor ring. As such, these types of sensors are generally not affected by this code.

In practical terms, a reference voltage can be thought of as a given sensor’s “power supply” that is delivered to that sensor by the PCM. Depending on the sensor and the circuit it controls/monitors, factors such as temperature and pressure alter the reference voltage as the sensors’ resistance changes in response to changing conditions.

For instance, as the transmission fluids’ temperature rises, the resistance of the Transmission Fluid Temperature sensor may increase (in some cases, the resistance decreases), thus allowing less current to pass through the sensor. The PCM interprets the changes in resistance from a sensor (the Transmission Fluid Temperature sensor in this case), as degrees of temperature, and it is this voltage (known as the signal voltage) that is used by the PCM to make changes or adaptations to a particular system, or to recognize a condition that requires an intervention, such as initiating a fail-safe mode or other preventive measure to prevent damage to components.

Thus, if an open circuit occurs in reference voltage circuit “A”, all the sensors, control modules, and sub-systems that share circuit “A” may be affected, but in many cases, code P0641 will be accompanied by other codes that narrow down the general area in which the open circuit is present. This usually takes the form of other codes that refer to a specific sensor or system, but note that this is not always the case. However, when code P0641 is present, whether other codes are shown or not, it always means that an open circuit exists in reference voltage circuit “A”. Be aware though that while on some applications this code will be stored on the first failure cycle, other applications may require several failure cycles before the code will set and a warning light is illuminated.

The image below shows the typical appearance of wiring that had rubbed through against engine components. While this is a common cause of open circuits that can set code P0641, it must be noted that in many cases, the cause of the open circuit might be buried deep in a wiring harness, where it might never be found unless the entire harness is removed and inspected/tested.

What are the common causes of code P0641 ?

Common causes of code P0641 could include the following-

• Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
• Defective sensors
• Defective control modules
• Unlike with most other codes, a defective PCM is a distinct possibility

What are the symptoms of code P0641 ?

In cases where reference voltage circuit “A” elates to the transmission/drive train, typical symptoms could include the following-

• Gearshifts may be harsh, delayed, erratic, unpredictable, or the transmission may not shift at all
• Transmission may not shift between pre-programmed modes
• Transmission may not shift between 2WD and AWD modes
• Transfer case may not shift between high and low ranges
• FWD hubs may not engage
• Speedometer/odometer operation may be erratic, or may not work at all
• Front or central differentials may not engage

NOTE: Be aware that the severity of one or more symptoms may depend on both the application, and the particular sensor/control module(s) that is affected by the open circuit.

How do you troubleshoot code P0641 ?

SPECIAL NOTES: Since many applications require multiple drive cycles with a fault (code P0641) present before a warning light will be illuminated, it is important to allow the PCM to enter “Readiness Mode” before a repair of his code can be considered as successful. Therefore, it is important to clear all codes after each repair step, and to operate the vehicle normally before rescanning to see if the code returns. If the PCM does not enter “Readiness Mode”, the fault is still present; conversely, if the PCM does enter “Readiness Mode” after repairs are complete, the repair can be considered successful. END OF SPECIAL NOTES.   

NOTE: Be aware that since code P0641 could involve open circuits in the CAN (Controller Area Network) bus system, access to an oscilloscope and a wave form library may be required to diagnose code P0641 accurately. Therefore, if the diagnostic/repair information presented here does not resolve the problem and an oscilloscope and reference data are not available, refer the vehicle to the dealer or other competent repair facility for professional diagnosis and repair.

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

NOTE: If other codes are present, note the order in which they were stored. Codes that follow P0641 will be stored as a result of P0641; codes that precede P0641 may have caused or contributed to P0641, and as such, these codes must be investigated and resolved before an attempt is made to diagnose P0641. Failure to do this will almost certainly result in a misdiagnosis, wasted time, and the unnecessary replacement of parts and components.

 Step 2

If no other codes are present, refer to the manual to determine which sensors, control modules, and systems are served by reference voltage circuit “A”, since manufacturers do not always follow convention when it comes to labelling parts, circuits, and sensors. Also determine the routing, color-coding, and function of all relevant wiring to avoid testing the wrong circuits and/or sensors later on.

NOTE: Be sure to refer to the manual to locate all system grounds, as well as all ground connection points relating to reference voltage circuit “A”, since loss of ground is a common cause of open circuits.   Also be sure to investigate the relationship between code P0641 and all codes that precede it, since resolving these codes might resolve P0641 as well. This is particularly true where one or more codes refer to a specific sensor or subsystem that obtains a reference voltage through circuit “A”.

Step 3

Start the procedure by performing a thorough visual inspection of all relevant wiring, and be sure to find and inspect all related fuses and fusible links. Replace blown fuses with their exact equivalents, but bear in mind that while blown fuses most commonly result from short circuits, fuses do not always need a reason to blow. Long use can weaken a fuse to the point where it blows the next time current flows through it.

Nonetheless, inspect all wiring for signs of damage. Look for damaged, burnt, chafed, shorted, disconnected, or corroded wiring and/or connectors. Make repairs or replace wiring as required. Clear all codes after repairs are complete, before operating the vehicle and rescanning the system to see if the code returns. Refer to the SPECIAL NOTES at the start of the Troubleshooting section.

Step 4

If no visible damage to wiring is found, prepare to perform resistance, reference voltage, continuity, and ground integrity checks on all relevant wiring and sensors. Be sure however to disconnect all affected sensors from the PCM and other control modules to prevent damage to one or more control modules during this step.

Refer to the manual on the correct procedure to follow to test the reference voltage circuit “A” at the PCM terminal to see if the PCM actually delivers the correct reference voltage at this point. If it does not, replace the PCM.

If the correct reference voltage is present at the PCM connector, follow the circuit through each sensor and control module that shares the circuit. However, be sure to perform this test with either KOER or KOEO as the case may require for each sensor. This is often the easiest and quickest way to isolate the problem, especially in cases where no other codes are present, or the scanner being used cannot diagnose CAN bus system faults.

NOTE: If the open circuit is located during Step 4, the better option is to replace the relevant harness instead of attempting to repair wiring. Clear all codes after repairs are complete, before operating the vehicle and rescanning the system to see if the code returns. Refer to the SPECIAL NOTES at the start of the Troubleshooting section.

Step 5

If the reference voltage circuit checks out, continue by performing resistance, ground integrity, and continuity tests on the other circuits in the system. Compare all obtained readings with the values stated in the manual, and make repairs or replace wiring as required to ensure that all electrical values fall within the ranges specified by the manufacturer.

NOTE #1: Be sure to test the internal resistance of each sensor that shares reference voltage circuit “A”, since sensors always from part of their control circuits. Replace any sensors that do not conform to the manufacturers’ specifications.

NOTE #2: Pay particular attention to the condition of connectors during this step. Physically check each connector for the presence of corrosion or poor contacts, and replace all and any connectors that are in a less than perfect condition. Clear all codes after repairs are complete, before operating the vehicle and rescanning the system to see if the code returns. Refer to the SPECIAL NOTES at the start of the Troubleshooting section.

Step 6

The steps outlined above will resolve the problem in nine out of every ten instances of code P0641. However, if the above steps do not resolve the issue, suspect an intermittent fault, or as faulty control module, but bear in mind that control modules can generally not be tested on a DIY basis, if they can be tested at all.

At this point, it becomes necessary to test the operation of each sensor with an oscilloscope, but this requires access to a complete wave form library for reference purposes. If such equipment and reference data are not available, continuing the diagnostic procedure is NOT recommended, since incorrect testing procedures from this point onwards can cause serious, if not fatal damage to the application’s electrical system, and/or to one or more control modules.

With regard to intermittent faults, bear in mind that these types of faults can be extremely challenging and time consuming to find and repair, and especially if the fault is in the CAN bus system. Professional mechanics often simply remove and replace the suspect wiring harness, since looking for an intermittent fault in the CAN bus system is akin to looking for a very small needle in hundreds of very big haystacks.

Codes Related to P0641

• P0642 – Relates to “Sensor Reference Voltage “A” Circuit Low”
• P0643 – Relates to “Sensor Reference Voltage “A” Circuit High”

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