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U0634 Lost Communication With Hybrid/EV Battery Pack Coolant Temperature Sensor A

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

Liquid-cooled traction batteries check how warm the coolant is where it enters or leaves the pack to decide whether to cool or heat. Where that sensor is a digital node rather than a plain thermistor, the battery or thermal controller can detect when its messages stop, and U0634 records exactly that for sensor A. No manufacturer documents this code publicly; GM, for instance, reads its battery coolant sensors as analog thermistors, with P0C42 to P0C46 for their circuit faults. For hybrids CARB requires every electronic input used for battery thermal management to be monitored like any comprehensive component, so a MIL is to be expected there. Without the reading the controller must estimate pack temperature and may reduce charge or drive power.

Likely causes

Data, supply or ground wire to sensor A damaged

Connector at the battery coolant line corroded or loose

Sensor A electronics failed

Battery or thermal management controller fault

Symptoms

Hybrid or battery warning lamp, on hybrids also the MIL
Reduced charging power or limited electric performance

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 avehicle bus wiring

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