What does it 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.