U0639
Lost Communication With Hybrid/EV Electronics Coolant Temperature Sensor B
The vehicle bus reports loss of communication with smart HV power-electronics coolant temperature sensor B. Inverter/DC-DC cooling switches to a substitute value; derating may follow.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: medium
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Common causes
Ordered by how often each turns out to be the cause.
- Bus wiring open or shortedCommon
- Module without power or groundPossible
- Module failedPossible
Parts involved
- Hv Electronics Coolant Temp Sensor B
- Vehicle Bus Wiring
U0639 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.