Damaged, corroded, or disconnected wiring at the coolant temperature sensor B connector; Open or shorted signal circuit between the sensor and its reporting control module; Failed hybrid/EV electronics coolant temperature sensor B; Faulty control module that broadcasts the sensor message on the communication bus; Low battery voltage or poor grounds causing intermittent module communication dropouts; Water intrusion or heat damage in the harness near the hybrid electronics cooling circuit; Aftermarket accessories or improper prior repairs disturbing the data bus wiring

DTC codes / U06xx · Network communication / U0639

U0639Lost Communication With Hybrid/EV Electronics Coolant Temperature Sensor B

U0639 is a network communication fault indicating that a control module on the vehicle's data bus has stopped receiving valid messages from the hybrid or electric vehicle electronics coolant temperature sensor B circuit. The computer did not detect an out-of-range temperature reading; instead, it logged a loss of the periodic serial data message that the sensor's reporting module normally broadcasts.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaNetwork communication

Symptoms you may notice

Illumination of the check engine light, service vehicle soon message, or hybrid system warning lamp
Often no symptoms besides the warning light, with the vehicle driving normally
Reduced hybrid system power or a forced limp-home mode in some vehicles
Electric cooling fans running continuously or not engaging at all
Inaccurate or missing coolant temperature display on the instrument cluster
Possible no-start condition if the hybrid control module refuses to close the contactors

Common causes, most likely first

Damaged, corroded, or disconnected wiring at the coolant temperature sensor B connector
Open or shorted signal circuit between the sensor and its reporting control module
Failed hybrid/EV electronics coolant temperature sensor B
Faulty control module that broadcasts the sensor message on the communication bus
Low battery voltage or poor grounds causing intermittent module communication dropouts
Water intrusion or heat damage in the harness near the hybrid electronics cooling circuit
Aftermarket accessories or improper prior repairs disturbing the data bus wiring

How to diagnose and fix it

Verify battery state of charge and perform a voltage drop test on the main grounds before any parts replacement
Inspect the coolant temperature sensor B connector and harness for corrosion, pushed pins, or chafed insulation
Measure resistance across the sensor and compare to the manufacturer's temperature-resistance chart
Check for continuity and short-to-ground on the signal and return circuits between the sensor and its module
Use a scan tool to confirm whether other modules on the same bus are also missing and to read live sensor data
Perform a wiggle test on the harness while watching live data to expose intermittent connection faults
Repair or replace the faulty sensor, pigtail, or module only after electrical testing confirms the failure

Why this code is rarely the sensor itself

U0639 is a communication code, so the sensor may be reporting perfectly while the module that broadcasts its message has simply gone offline.
A weak 12-volt battery is a frequent hidden cause because hybrid control modules drop off the bus when supply voltage sags during cranking or load spikes.
Technicians often replace the coolant temperature sensor only to find the real fault is a corroded terminal in the harness connector or a chafed wire near the cooling fan shroud.
This code can set alongside other U-codes when a single module or a shared bus segment fails, so always check whether multiple communication faults point to one common node.
Some vehicles set U0639 after a low-voltage event or module reflash and will not clear until the module completes a full sleep-wake cycle.

Can I keep driving with U0639?

The vehicle is usually safe to drive short distances because the fault is a communication loss rather than an overheating condition, but avoid long trips or heavy electrical loads until the circuit is repaired. If the hybrid system warning lamp is on or power is reduced, have the vehicle inspected promptly to prevent cooling system damage.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

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