The U0639 code indicates that the vehicle's control network has lost communication with the hybrid/electric vehicle (EV) electronics coolant temperature sensor B. This is a network communication fault, meaning the relevant module cannot receive data from the sensor, which may lead to default temperature strategies and reduced hybrid performance.
The sensor itself may have failed, causing loss of signal.
Damage, corrosion, or looseness in the sensor's wiring harness can interrupt communication.
A blown fuse, open circuit, or poor ground to the sensor can prevent it from operating.
A short, open, or high resistance in the controller area network can block messages from the sensor.
Rarely, the module that receives the sensor data may itself have a hardware or software issue.
It is not advisable; the fault can cause the hybrid system to operate in a limited‑performance mode and may lead to improper engine temperature regulation.
The vehicle may still be drivable, but you may experience reduced hybrid efficiency and should have the issue diagnosed promptly.
In most cases, the fault lies with the sensor, wiring, or a network issue; PCM replacement is rarely required and should be considered only after other causes are ruled out.
If the sensor is the root cause, replacing it will clear the code, but you must also repair any damaged wiring or connectors to prevent the issue from returning.