New Holland 3521 – Engine control module: Nitrogen Oxide Assessment Error - Unreliable Signal

The Engine Control Unit (ECU) has detected a failure in the nitrogen oxide (NOx) estimation process, indicating that the NOx assessment signal is unreliable. This fault affects the Selective Catalytic Reduction (SCR) system's ability to properly manage Diesel Exhaust Fluid (DEF) injection, which can disrupt emissions control and lead to increased emissions and engine performance issues.

• T8.xxx Series
• T7 Series
• T9 Series
• T6 Series
• New Holland Construction equipment
• Dozer models

1. 1

   Retrieve all fault codes using a diagnostic scanning tool, specifically looking for related NOx sensor codes (e.g., 3528 for plausibility, 3529/3530 for open/short circuits, or 3532 for sensor readiness)

2. 2

   Inspect the NOx sensor wiring harness and connectors for any signs of damage, corrosion, or loose connections

3. 3

   Check Diesel Exhaust Fluid (DEF) quality, as poor or contaminated DEF can trigger SCR-related faults

4. 4

   Monitor live data using a diagnostic tool to verify NOx sensor readings (upstream should read around 500 ppm, decreasing to around 150 ppm downstream after the SCR catalyst)

1. 1

   Address related fault codes

   If other fault codes are present, especially those indicating a clear electrical fault with a NOx sensor (such as an open or short circuit), address those first.

2. 2

   Test the NOx sensor

   If wiring and DEF quality are confirmed good, test the NOx sensor wiring with a multimeter or use a specialized NOx sensor tester to determine if the sensor has failed.

3. 3

   Replace the NOx sensor

   If the sensor is faulty, replace it with an OEM sensor. Ensure the exhaust pipe threads are clean of carbon deposits before installing the new sensor.

4. 4

   Clear codes and reset system

   Clear the fault codes using a diagnostic tool. Perform a system reset or drive cycle as required by the specific SCR system to recalibrate.