This code (P0641 OBDII) is thrown when the PCM notices an open circuit in the reference voltage circuit “A”. The reference voltage circuit “A” is the circuit that supplies a voltage to the sensors that control the transmission train. All the sensors and modules in this circuit are interconnected through the CAN bus system (Control Area Network).
If there is an open circuit in the reference voltage circuit “A”, all these sensors, modules and subsystems that share this circuit may be faulty. Usually, this code is accompanied by other fault codes.