U0646: Lost Communication With Motor Electronics Coolant Pump A

What it means

A control module has stopped receiving expected network messages from motor electronics coolant pump A. The effect on motor electronics cooling 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

Motor electronics coolant pump A circulates coolant to help control the temperature of the vehicle’s motor electronics. U0646 sets when a control module stops receiving expected network messages from pump A, leaving its commanded state or status unavailable and potentially limiting vehicle performance.

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
• Motor electronics cooling or vehicle performance may be affected

How to diagnose it

1. Scan all modules and save freeze-frame data, noting any motor-electronics temperature, cooling, or pump codes alongside U0646.
2. Check live data for pump A status and motor-electronics temperatures, and look for an absent pump response or rising temperature.
3. With the vehicle safely powered down as specified by the manufacturer, inspect pump A’s connector and accessible network wiring for damage or corrosion.
4. Check pump A’s module power and ground circuits against the manufacturer’s specification.
5. Test the network wiring to pump A for opens or shorts using the vehicle wiring information.
6. If power, ground, and network wiring pass, verify pump communication and operation before replacing the pump module.

Common fixes

• Reconnect or clean the motor electronics coolant pump A connector.
• Repair damaged network wiring to pump A.
• Restore the pump module’s power or ground supply.
• Replace pump A if its circuits and network connection are sound but it remains unresponsive.

Can I keep driving?

With a cooling-system warning, rising temperature, or reduced-power message, stop in a safe place and follow the vehicle’s instructions; continued driving can overheat motor electronics. Do not open a hot cooling system, and arrange prompt diagnosis.