U02A2

Lost Communication With Reductant Control Module

U02A2 is a generic OBD-II network diagnostic trouble code: Lost Communication With Reductant Control Module. It is logged by the engine control unit when the network monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
U02A2
Group
Network
System
Network
Severity
Warning (MIL on, possible limp mode)
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What U02A2 means

U02A2 is stored by a receiving module (typically the ECM or TCM) when it detects that expected CAN bus messages from the reductant control module (also referred to as the SCR dosing control unit) have not been received within the required time window. The reductant control module manages DEF pump operation, dosing injector control, and related sensor monitoring, and it broadcasts status data on the vehicle network at a defined rate.

Loss of communication with the reductant control module prevents the ECM from receiving dosing feedback, pressure data, and system status, which may cause the ECM to suspend reductant dosing commands or enter a default strategy. Sustained communication loss will typically lead to SCR system performance faults and emissions-related warnings including possible torque deration.

Diagnosis should start with confirming the network topology, checking CAN bus wiring between the reductant module and the network, verifying module power and ground supplies, and ruling out a module that has simply lost power rather than a true bus fault. A CAN bus waveform check at the diagnostic port can help confirm whether the issue is isolated to the reductant module or is a wider bus problem.

Common causes

Most-frequently reported root causes when U02A2 is logged.

  • 1
    Loss of power or ground to the reductant control module.
  • 2
    Blown fuse supplying the reductant control module.
  • 3
    Open or short circuit in the CAN bus wiring to the reductant module.
  • 4
    Corroded or damaged connector at the reductant control module.
  • 5
    Failed reductant control module not transmitting on the bus.
  • 6
    CAN bus termination resistor fault causing signal degradation.
  • 7
    Interference from a recently installed or faulty aftermarket device on the CAN bus.

Symptoms drivers notice

MIL and possibly other warning lights illuminated.
SCR system inoperative with no reductant dosing.
Torque deration or emission-related power limitation after sustained communication loss.
Scan tool may be unable to communicate with or display live data from the reductant module.
Multiple other modules may log U-code communication faults if the CAN bus is compromised.

How to diagnose U02A2

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all DTCs from all available modules; widespread U-codes suggest a bus-level fault rather than a module fault.
  2. 2
    Verify battery voltage and check the fuse for the reductant control module power supply.
  3. 3
    Inspect the reductant control module connector and CAN bus wiring for damage, corrosion, or disconnection.
  4. 4
    Attempt to communicate directly with the reductant module via the scan tool to confirm whether it is visible on the bus.
  5. 5
    Measure CAN bus voltage levels at the OBD-II port to verify normal CAN high and CAN low differential signal.
  6. 6
    Check CAN bus termination resistance with all modules disconnected to verify integrity of the network.
  7. 7
    If bus integrity is confirmed and the module has power but does not respond, suspect a failed reductant control module.

Related network codes

Frequently asked questions

Is U02A2 always stored in the ECM, or can it be stored in other modules?

U-codes are stored in the module that lost communication, not in the module that went offline. U02A2 can therefore appear in the ECM, the instrument cluster, or any other module expecting messages from the reductant control unit.

Could a dead battery cause U02A2?

Yes. Voltage drop during starting or after a deep discharge can cause module resets or failed communication, setting U-codes that clear after the battery is recharged and the system is re-initialized.

How quickly will torque deration occur after U02A2 sets?

Deration strategies vary by OEM calibration. Most systems apply a warning period of one or more drive cycles before enforcing a power limitation, but some may derate immediately if dosing cannot be confirmed.

Does U02A2 mean the reductant module needs replacement?

Not necessarily. Power supply issues, wiring faults, and connector corrosion are more common causes. Module replacement should only be considered after all electrical faults and power supply issues have been ruled out.

Disabling U02A2 in software

RaceTune can permanently disable U02A2 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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