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

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

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.

Likely causes

Harness to sensor B damaged, for example near hoses or brackets

Moisture or bent pins in the sensor B connector

Sensor B itself no longer transmits

Bus master controller fault

Symptoms

Hybrid or EV system warning lamp
Correlation checks between the electronics coolant sensors disabled
Occasional power limitation on long climbs or fast charging

Repair estimate

DifficultyHardWorkshop job

Parts and labour€80–600

Time0.5–2.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

hv electronics coolant temp sensor bvehicle bus wiring

References: law.cornell.edu, nxp.com.