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U0635 Lost Communication With Hybrid/EV Battery Pack Coolant Temperature Sensor B

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

Sensor B is the second battery coolant temperature point, commonly placed on the opposite side of the pack from sensor A so the controller can see how much heat the coolant picks up. U0635 means sensor B no longer delivers digital messages. No public manufacturer document uses the code. With sensor A still working the controller keeps one reference but loses the temperature difference across the pack, which it uses to size pump speed and chiller or heater demand. CARB's hybrid rules count such inputs among the battery thermal management parts that must be monitored, and also ask for a functional check of cooling where feasible. The analog circuit faults of the same sensor are P0CD5 to P0CD9. Check the wiring branch to B before replacing parts.

Likely causes

Wiring branch to sensor B open, shorted or chafed

Moisture or corrosion at the sensor B connector

Sensor B failed

Symptoms

Battery or hybrid system warning
Battery cooling runs more often or charging power is reduced

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 battery pack coolant temp sensor bvehicle bus wiring

References: law.cornell.edu, en.wikipedia.org, nxp.com.