Damaged or corroded wiring harness at the coolant control valve connector; Failed coolant control valve B actuator or internal position sensor; Blown fuse or poor ground connection supplying the valve circuit; CAN bus communication fault between the hybrid control module and the valve, such as an open or shorted data line; Software glitch or outdated calibration in the hybrid control module; Coolant contamination or mechanical binding of the valve causing it to draw excessive current and drop offline

DTC codes / U06xx · Network communication / U0637

U0637Lost Communication With Hybrid/EV Battery Pack Coolant Control Valve B

U0637 is a network communication code indicating that a control module on the vehicle's Controller Area Network (CAN) bus has lost contact with the hybrid or electric vehicle battery pack coolant control valve B. The module that monitors this valve, typically the hybrid powertrain control module, has stopped receiving expected messages or feedback from the valve's control circuit, so it sets this code and may default to a fail-safe cooling strategy.

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 light
Often no symptoms besides the warning light, especially if the valve fails in a default position
Reduced hybrid system performance or a forced limp mode in some vehicles
Inability to charge the high-voltage battery or unexpected charging interruptions
Coolant temperature warnings or fan running excessively due to lost valve control
Overheating of the hybrid battery pack in severe cases, leading to power reduction

Common causes, most likely first

Damaged or corroded wiring harness at the coolant control valve connector
Failed coolant control valve B actuator or internal position sensor
Blown fuse or poor ground connection supplying the valve circuit
CAN bus communication fault between the hybrid control module and the valve, such as an open or shorted data line
Software glitch or outdated calibration in the hybrid control module
Coolant contamination or mechanical binding of the valve causing it to draw excessive current and drop offline

How to diagnose and fix it

Perform a visual inspection of the coolant control valve B connector and wiring for corrosion, chafing, or loose pins
Check all fuses and relays related to the hybrid cooling system and verify battery voltage at the valve connector
Use a scan tool to read live data and confirm whether the valve responds to commanded positions; check for other communication codes on the same bus
Measure resistance and voltage on the CAN bus lines at the valve and at the hybrid control module to isolate an open or short circuit
Test the valve actuator directly with a bidirectional scan tool or by applying known good signals to confirm mechanical or electrical failure
If wiring and power are good, replace the coolant control valve B and clear the code, then perform a relearn or calibration procedure as required
Update the hybrid control module software if a technical service bulletin addresses this communication fault

What most techs overlook on U0637

A loose or corroded ground strap on the hybrid transaxle or battery pack can cause intermittent loss of communication with the coolant valve, mimicking a failed valve.
The code can set from a low 12-volt battery or charging system issue because the valve module may drop offline during cranking or voltage spikes.
Some vehicles use a shared coolant control valve for both battery and cabin heating, so a fault in the cabin loop can trigger U0637 even when battery cooling seems fine.
After replacing the valve, many models require a coolant bleed procedure and a valve position relearn; skipping this step will set the code again immediately.
U0637 often appears alongside other U-codes for the same bus, so always check for a common power or ground fault before condemning the valve itself.

Can I keep driving with U0637?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy acceleration, long trips, or high ambient temperatures because the hybrid battery may not cool properly. If the vehicle enters limp mode or shows a red battery temperature warning, stop safely and have it towed.

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.