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

What it means

A control module has stopped receiving expected network messages from hybrid/EV electronics coolant temperature sensor A. The effect on electronics 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 electronics coolant temperature sensor A monitors coolant temperature in a circuit serving vehicle electronics, with its reading passed through a networked module. U0638 sets when the receiving module stops getting expected messages from sensor A, leaving that electronics-cooling temperature input unavailable.

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
• Coolant temperature data may be unavailable

How to diagnose it

1. Save freeze-frame data and scan all modules for electronics cooling, network, and module power codes.
2. Check live data for electronics coolant temperature sensor A and compare it with other available coolant temperature readings.
3. Inspect sensor A's low-voltage wiring and connectors for chafing, corrosion, loose pins, or coolant intrusion.
4. Check the communicating module's power and ground using the wiring diagram and manufacturer's specifications.
5. Test the network wiring for opens or shorts and evaluate the reporting module under the manufacturer's hybrid/EV procedure if the circuit and supplies are sound.

Common fixes

• Repair damaged communication wiring or loose terminals on sensor A's circuit.
• Clean or replace corroded sensor or reporting-module connector terminals and restore seals.
• Repair a verified power or ground fault to the module carrying sensor A data.
• Replace the communicating module only after its network wiring and supplies have been confirmed good.

Can I keep driving?

Avoid prolonged high-load driving if an electronics cooling warning appears or the vehicle limits power. Loss of sensor A data can impair monitoring of electronics cooling, so stop and follow the vehicle's warning guidance if temperatures rise or a thermal warning is shown.