U0639 code meaning
Lost Communication With Hybrid/EV Electronics Coolant Temperature Sensor B
Network system Generic code U06xx codes Check engine light
What U0639 means
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.
U0639 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyHard
- Typical labor0.5 to 2.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0639
- Hybrid or EV system warning lamp
- Correlation checks between the electronics coolant sensors disabled
- Occasional power limitation on long climbs or fast charging
Common causes of U0639
- Most 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
- Less likelyBus master controller fault
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Hv electronics coolant temp sensor bVehicle bus wiring
What each part of U0639 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 39 | The exact fault in that area. |
What to do next
- Note U0639 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U0639
What does U0639 mean?
U0639 means "Lost Communication With Hybrid/EV Electronics Coolant Temperature Sensor B". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with U0639?
Drive gently and only to a workshop. With U0639 the car can go into limp mode (reduced power) to protect itself.
Will U0639 fail an emissions test?
Yes, it can. U0639 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is U0639 to fix?
Difficulty: hard. A typical repair takes 0.5 to 2.5 hours of work. This is a job for a workshop.
Will U0639 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.