U0639Lost Communication With Hybrid/EV Electronics Coolant Temperature Sensor B
- Generic OBD-II code · Network
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- A second temperature point in the electronics cooling loop, labelled B, lets the controller compare inlet and outlet or watch a separate branch, and the pair is checked against each other by P2D2A. U0639 means that sensor B, read as a digital bus participant, has gone quiet while the bus itself is still working. No manufacturer documents U0639 publicly, so its exact mounting point stays vehicle-specific. CARB treats such inputs of inverter thermal management as monitored components under (e)(15.2.3)(B)(ii). While sensor B is missing, any plausibility check between A and B cannot run, and the cooling strategy falls back to sensor A alone or to a model. Wiring faults of an analog sensor B would appear as P2B20 to P2B23.
- What you may notice
- Hybrid or EV system warning lamp
- Correlation checks between the electronics coolant sensors disabled
- Occasional power limitation on long climbs or fast charging
- Likely causes
- LikelyHarness to sensor B damaged, for example near hoses or brackets
- PossibleMoisture or bent pins in the sensor B connector
- PossibleSensor B itself no longer transmits
- RareBus master controller fault
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €80 – €600
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.