U02A2: Lost Communication With Reductant Quality Module

What it means

The vehicle bus reports loss of communication with the reductant (DEF/AdBlue) quality sensor module. The SCR system cannot verify urea concentration; emissions countermeasures may activate.

At a glance

• Usually lights the warning light of the system that lost communication
• Affects emissions; the car can fail an emissions test
• The engine or gearbox may go into limp mode

How it works

The reductant quality module reports the DEF/AdBlue concentration used by the SCR emissions system. The vehicle network sets U02A2 when other controllers stop receiving messages from that module, so they cannot confirm the fluid quality and may limit emissions-system operation.

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 for communication codes and record freeze-frame data to see whether U02A2 appeared with other network faults or a low-voltage event.
2. Check the DEF/AdBlue quality warning and related SCR data for missing or implausible readings that point to loss of the quality module's messages.
3. Inspect the quality module connector and nearby harness for loose pins, corrosion, water, chafing, or damage.
4. With the vehicle's wiring information, check the quality module's power and ground under the conditions specified by the manufacturer.
5. Test the network wiring at the quality module for opens, shorts, or poor connections, following the manufacturer's network test procedure.
6. If power, ground, and network wiring pass, confirm the fault follows the quality module before replacing it.

Common fixes

• Reconnect or repair a loose, corroded, or damaged quality-module connector.
• Repair an open or short in the bus wiring to the quality module.
• Restore the module's failed power or ground connection.
• Replace the reductant quality module if its supplies and network circuit pass testing.

Can I keep driving?

The vehicle may remain driveable, but SCR warnings or emissions-related limits can appear if DEF quality cannot be verified. Avoid extended driving if a countdown or reduced-power warning appears, and address the fault promptly.