U0635: Lost Communication With Hybrid/EV Battery Pack Coolant Temperature Sensor B

What it means

A control module has stopped receiving expected network messages from hybrid/EV battery pack coolant temperature sensor B. The effect on battery thermal management depends on the vehicle's system design.

At a glance

• Usually lights the warning light of the system that lost communication
• The engine or gearbox may go into limp mode

How it works

Hybrid/EV battery pack coolant temperature sensor B reports a battery cooling temperature through a networked module. U0635 means expected messages from sensor B are missing, specifically affecting the B input rather than sensor A.

Common causes

Most likely first.

1. Bus wiring open or shorted
2. Module without power or ground
3. Module failed

Symptoms

• Warning light or message related to the affected system
• Battery temperature data may be unavailable

How to diagnose it

1. Record freeze-frame data and scan all modules for battery cooling, network, high-voltage system, and module supply codes.
2. Read live data for battery coolant temperature sensor B and compare it with sensor A and other battery temperature values if available.
3. Inspect sensor B's low-voltage harness and connectors for abrasion, corrosion, loose pins, or coolant intrusion, avoiding high-voltage parts.
4. Verify power and ground at the module carrying sensor B's messages according to the wiring diagram and manufacturer's specifications.
5. If the fault remains, test the network and assess the communicating module using the manufacturer's hybrid/EV procedures.

Common fixes

• Repair an open, short, or damaged low-voltage communication wire for sensor B.
• Clean or replace corroded or loose terminals at sensor B's connector or reporting module.
• Restore a verified power or ground fault at the module serving sensor B.
• Replace the communicating module only after its network wiring and electrical supplies are confirmed.

Can I keep driving?

Avoid heavy acceleration and fast charging if a battery thermal-management warning appears or the vehicle limits power. Loss of sensor B data can reduce battery temperature monitoring, so stop and follow the vehicle's guidance if a temperature warning is displayed.