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U0638 Lost Communication With Hybrid/EV Electronics Coolant Temperature Sensor A

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

The coolant loop that serves the inverter and other high-voltage electronics needs its own temperature reading so the controller can size pump speed and decide when to derate. When sensor A on that loop is a bus node that sends its value digitally, for instance over LIN, the receiving controller notices when frames stop coming, and U0638 records that silence. No manufacturer documents this code publicly. CARB 13 CCR 1968.2 (e)(15.2.3)(B)(ii) requires every electronic input that serves inverter cooling to be checked like other input components, so a MIL is expected on hybrids. An analog sensor of the same position reports wiring faults through P0CEE to P0CF2 instead. Without a reading the system usually assumes a warm loop and limits electric power.

Likely causes

Data, supply or ground line to sensor A open, shorted or chafed

Corroded or loose connector at the electronics coolant line

Internal electronics of sensor A failed

Fault or outdated software in the controller that polls the sensor

Symptoms

Hybrid system warning, on hybrids usually the MIL as well
Reduced electric drive power, especially under load or in hot weather
Electronics coolant pump running continuously at high speed

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 avehicle bus wiring

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