U0639: Lost Communication With Hybrid/EV Electronics Coolant Temperature Sensor B

What it means

A control module has stopped receiving expected network messages from hybrid/EV electronics coolant temperature sensor B. The effect on electronics thermal management depends on the vehicle's system design.

At a glance

• Usually lights the warning light of the system that lost communication
• The engine or gearbox may go into limp mode

How it works

Hybrid/EV electronics coolant temperature sensor B reports the coolant temperature used to protect high-voltage electronics. The vehicle network carries its readings to the control modules that manage cooling. U0639 sets when a module no longer receives the expected messages from sensor B, so its temperature reading may be unavailable.

Common causes

Most likely first.

1. Bus wiring open or shorted
2. Module without power or ground
3. Module failed

Symptoms

• Warning light or message related to the affected system
• Coolant temperature data may be unavailable

How to diagnose it

1. Scan all modules for related communication codes and review freeze-frame data to see when sensor B stopped reporting.
2. Compare live data for electronics coolant temperature sensor B with other available temperature readings to confirm that its value is missing or stale.
3. Inspect sensor B’s network wiring and connectors for looseness, corrosion, chafing, or damage, using the vehicle wiring information to identify the correct circuit.
4. Check the sensor or associated module’s power and ground against the manufacturer’s specification.
5. Test the network circuit for opens or shorts, then determine whether the sensor’s reporting module has failed.

Common fixes

• Reconnect or repair a loose or damaged sensor B network connector.
• Repair an open or short in the network wiring to sensor B.
• Restore the sensor or its module’s power or ground supply.
• Replace the failed sensor B reporting module if circuit checks confirm the fault is internal.

Can I keep driving?

Avoid demanding driving if the hybrid/EV system displays a temperature or power warning, or if performance is limited; electronics cooling may be impaired and the vehicle may enter reduced-power operation. Stop safely if a temperature warning appears, and arrange diagnosis before continued use.