U0631: Lost Communication With Engine Coolant Temperature Sensor 5

What it means

A control module has stopped receiving expected network messages from engine coolant temperature sensor 5. The effect on engine temperature monitoring and control depends on the vehicle's system design.

At a glance

• Usually lights the warning light of the system that lost communication
• Affects emissions; the car can fail an emissions test

How it works

Engine coolant temperature sensor 5 provides temperature information through a networked module on vehicles equipped with this sensor arrangement. U0631 sets when the receiving module no longer gets the expected messages specifically from sensor 5, so its temperature data may be 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
• Engine temperature data may be unavailable

How to diagnose it

1. Save freeze-frame information and scan all modules for related network, temperature, and module power codes.
2. View live data for engine coolant temperature sensor 5 and compare its reading with other temperature inputs during a cold start and warm-up.
3. Inspect sensor 5's harness and connectors for damage, corrosion, loose pins, or moisture.
4. Check power and ground at the module that relays sensor 5 data, using the wiring diagram and manufacturer's specifications.
5. Test the network wiring for opens or shorts according to the manufacturer's procedure, then investigate the communicating module if wiring and supplies are correct.

Common fixes

• Repair an open, short, or damaged section of the sensor 5 network wiring.
• Clean or replace loose or corroded terminals at sensor 5 or its communicating module.
• Restore a poor power or ground connection to the module serving sensor 5.
• Replace that module only after its communication wiring and electrical supplies have passed checks.

Can I keep driving?

The engine may still have other temperature readings, but loss of sensor 5 data can leave part of temperature monitoring unavailable. Drive gently and stop if a temperature warning appears or the engine runs hot, since an inaccurate temperature picture can delay protective action.