Damaged, corroded, or disconnected wiring or connector pins at the coolant temperature sensor; Open or shorted sensor signal circuit between the sensor and its reporting control module; Failed hybrid/EV electronics coolant temperature sensor itself; Faulty reporting module (such as the hybrid control module) that is not transmitting sensor data on the bus; Low voltage or poor grounds affecting the module that publishes the sensor message; CAN bus communication faults elsewhere on the network that disrupt the sensor message; Aftermarket accessories or poor previous repairs introducing interference or incorrect wiring

DTC codes / U06xx · Network communication / U0638

U0638Lost Communication With Hybrid/EV Electronics Coolant Temperature Sensor A

U0638 is a network communication code indicating that a control module on the vehicle's data bus has lost contact with the hybrid or electric vehicle electronics coolant temperature sensor 'A' circuit. The computer did not detect an out-of-range temperature reading; instead, it noticed that messages from that sensor or its reporting module stopped arriving within the expected time window.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaNetwork communication

Symptoms you may notice

Illumination of the check engine light or a dedicated hybrid system warning lamp
Often no symptoms besides the warning light, especially if the fault is intermittent
Reduced hybrid system power or a forced limp-home mode in some vehicles
Electric cooling fans running continuously or not running at all
Inaccurate or missing coolant temperature information on the dashboard or scan tool
Possible charging system or battery thermal management warnings if the sensor is shared with those systems

Common causes, most likely first

Damaged, corroded, or disconnected wiring or connector pins at the coolant temperature sensor
Open or shorted sensor signal circuit between the sensor and its reporting control module
Failed hybrid/EV electronics coolant temperature sensor itself
Faulty reporting module (such as the hybrid control module) that is not transmitting sensor data on the bus
Low voltage or poor grounds affecting the module that publishes the sensor message
CAN bus communication faults elsewhere on the network that disrupt the sensor message
Aftermarket accessories or poor previous repairs introducing interference or incorrect wiring

How to diagnose and fix it

Connect a scan tool and record all stored and pending codes in every module, not just the engine controller
Inspect the coolant temperature sensor connector and harness for corrosion, chafing, or loose pins
Check the sensor's resistance or voltage output against the manufacturer's specification while monitoring live data
Verify the reporting module has good power and ground connections and is communicating on the bus
Perform a voltage drop test on the sensor signal and return circuits to rule out high resistance
If the sensor tests bad, replace it and clear the code; if the module is not transmitting, diagnose the module or its bus wiring
After repairs, road test the vehicle through several warm-up cycles to confirm the code does not return

What most technicians miss on U0638

A U0638 can be set by a completely different module dropping off the bus, because the sensor message may be relayed through a gateway or hybrid controller rather than sent directly by the sensor.
The coolant temperature sensor on a hybrid or EV often shares a circuit with battery thermal management, so a fault here can cause charging or power-limiting symptoms that look unrelated to coolant.
Intermittent U0638 codes frequently trace to a connector that passes through a splash zone or near a heat source, where the pins look fine but lose contact only when hot or wet.
Replacing the sensor without checking the reporting module's power and ground is a common misdiagnosis, because a module with low voltage can stop transmitting without setting its own code.
Some vehicles require a relearn or adaptation after sensor replacement, and skipping it can leave the code stored even though the hardware is good.

Can I keep driving with U0638?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy acceleration, long trips, or towing because the hybrid system may limit power or cooling without warning. If the vehicle enters limp mode or shows a red hybrid warning, stop safely and arrange a tow.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.