U02A5: Lost Communication With Reductant Heater Control Module

What it means

The vehicle bus reports loss of communication with the reductant heater module. AdBlue lines and tank can no longer be thawed at low temperatures, blocking SCR operation in winter.

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

The reductant heater control module manages heating for the DEF/AdBlue tank and lines so fluid can thaw in cold weather. U02A5 sets when the vehicle network stops receiving messages from this module, preventing coordinated heater control and potentially leaving reductant frozen.

Common causes

Most likely first.

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

Symptoms

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

How to diagnose it

1. Scan all modules and capture freeze-frame data, noting ambient temperature and any other network or low-voltage codes.
2. Check live reductant heater status and temperature data to see whether the heater module is absent from the network or only reports a heater fault.
3. Inspect the heater module connector and nearby tank and line wiring for corrosion, water entry, chafing, or damage.
4. Check the module's power and ground using the manufacturer's wiring information and test conditions.
5. Test the network wires to the heater module for opens, shorts, or poor connections according to the manufacturer's procedure.
6. If power, ground, and network wiring pass, confirm module failure before replacing it.

Common fixes

• Reconnect or repair a loose or corroded heater-module connector.
• Repair damaged wiring around the reductant tank or heated lines.
• Repair an open or short in the module's network circuit.
• Restore the heater module's power or ground supply.
• Replace the reductant heater control module if its supplies and network circuit pass testing.

Can I keep driving?

In freezing conditions, the tank or lines may not thaw, which can prevent SCR operation and trigger emissions warnings or a countdown. Avoid relying on the vehicle for extended cold-weather trips until the heater communication fault is repaired.