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, when it detects that expected CAN bus messages from the Reductant Control Module have not been received within the required time window. On many diesel platforms this module is the reductant quality or dosing control unit within the selective catalytic reduction (SCR) system, responsible for monitoring diesel exhaust fluid and reporting related sensor and status data on the vehicle network at a defined rate.

Loss of communication with the reductant module prevents the ECM from receiving the feedback it relies on to manage SCR operation, which may cause it to suspend dosing commands or fall back to a default strategy. Sustained communication loss typically leads to SCR performance faults, emissions-related warnings, and, over one or more drive cycles, torque deration.

Diagnosis should start with confirming the network topology and checking the CAN bus wiring between the reductant module and the network, then verifying module power and ground supplies to rule out a module that has simply lost power rather than a true bus fault. A CAN bus waveform check at the diagnostic port helps confirm whether the issue is isolated to the reductant module or is a wider network problem. Retrieving codes from all modules distinguishes a single-node fault from a bus-level failure.

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
    Incorrect or corrupted module software preventing normal communication.

Symptoms drivers notice

MIL and possibly other warning lights illuminated.
SCR system inoperative with no reductant dosing feedback.
Torque deration or emission-related power limitation after sustained communication loss.
Scan tool unable to communicate with or display live data from the reductant module.
Multiple modules logging U-code communication faults if the 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 single module fault.
  2. 2
    Verify battery voltage and check the fuse for the reductant control module power supply.
  3. 3
    Inspect the reductant module connector and CAN bus wiring for damage, corrosion, or disconnection.
  4. 4
    Attempt to communicate directly with the reductant module to confirm whether it is visible on the bus.
  5. 5
    Measure CAN high and CAN low differential voltage at the OBD-II port to verify normal signaling.
  6. 6
    Check CAN bus termination resistance to verify network integrity.
  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 module.

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 often clear after the battery is recharged and the system re-initializes.

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 sooner 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, so module replacement should only be considered after all electrical and power supply faults 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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