U04A4

Lost Communication With NOx Sensor Module A

U04A4 is a generic OBD-II network diagnostic trouble code: Lost Communication With NOx Sensor Module A. 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
U04A4
Group
Network
System
Network
Severity
Warning (MIL on)
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What U04A4 means

U04A4 is stored by the ECM or emissions control module when it stops receiving expected CAN or SENT bus messages from NOx sensor module A (typically the upstream or pre-SCR NOx sensor). NOx sensors are intelligent devices with integrated electronics that calculate NOx concentration and oxygen content and transmit the results over the vehicle network rather than using a simple analog signal.

The upstream NOx sensor provides the ECM with real-time NOx data used to calculate required reductant dosing rates and to evaluate SCR catalyst conversion efficiency. Loss of this signal forces the ECM into a substitute dosing strategy that may be less precise, potentially resulting in under or over-dosing and associated performance or efficiency fault codes.

Diagnosis of U04A4 follows the same network fault methodology as other U-codes: confirm power and ground at the sensor, inspect the communication wiring (CAN or SENT line), and verify that the sensor is recognized by the scan tool. Mechanical damage to the sensor or its connector from proximity to hot exhaust components is a common real-world cause.

Common causes

Most-frequently reported root causes when U04A4 is logged.

  • 1
    Open or short circuit in the NOx sensor A CAN or SENT communication wire.
  • 2
    Loss of power or ground supply to the NOx sensor module.
  • 3
    Corroded, damaged, or thermally degraded NOx sensor connector.
  • 4
    Failed NOx sensor module A not transmitting data.
  • 5
    Heat damage to the wiring harness in the exhaust area adjacent to the sensor.
  • 6
    CAN bus fault affecting the segment to which the NOx sensor is connected.
  • 7
    Recent sensor replacement with an incompatible or unlearned sensor.

Symptoms drivers notice

MIL illuminated.
SCR dosing may revert to a default or open-loop strategy.
Possible companion SCR efficiency or dosing accuracy DTCs.
Scan tool may show no live data or no response from NOx sensor A.
Elevated NOx emissions due to imprecise dosing without sensor feedback.

How to diagnose U04A4

A typical diagnostic flow when this code is present.

  1. 1
    Scan all modules for DTCs and note whether U04A4 is isolated to one module or appears in multiple modules.
  2. 2
    Verify power supply voltage and ground quality at the NOx sensor A connector.
  3. 3
    Inspect the NOx sensor connector and harness for heat damage, melted insulation, or corrosion.
  4. 4
    Attempt to read live data from NOx sensor A with the scan tool to assess whether any communication exists.
  5. 5
    Check the communication wire (CAN or SENT) for continuity and shorts between the sensor and the control module.
  6. 6
    If wiring and power are intact, substitute with a known-good or correctly specified NOx sensor and retest.
  7. 7
    Clear codes and perform a drive cycle to confirm the fault does not return.

Related network codes

Frequently asked questions

What is NOx sensor module A versus module B?

On vehicles with dual NOx sensors, module A typically refers to the upstream (pre-SCR catalyst) sensor and module B to the downstream (post-SCR) sensor. The exact designation varies by OEM wiring diagrams.

Can I drive with U04A4 active?

Short-term driving is possible, but the SCR system will operate with reduced accuracy and emissions compliance cannot be guaranteed. Persistent driving risks further deration or catalyst efficiency faults.

Why do NOx sensors communicate digitally rather than using a simple voltage output?

NOx sensors perform complex electrochemical calculations internally. Digital communication allows them to transmit both NOx and lambda values along with sensor status and diagnostic information in a single connection.

Does U04A4 always mean the sensor itself has failed?

No. Wiring and power supply issues are very common causes. Always verify the circuit before condemning the sensor, as NOx sensors are expensive components.

Disabling U04A4 in software

RaceTune can permanently disable U04A4 — 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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