New Holland 3521 – Engine: NOx Estimation Failure

The Engine Control Module (ECM) has detected an issue with its internal software-based model for estimating Nitrogen Oxides (NOx) levels, indicating that the estimated NOx signal is not reliable. This estimation serves as a redundant check and baseline for the physical NOx sensors. When this model produces unreliable data, it can negatively affect the proper operation and efficiency of the engine's Selective Catalytic Reduction (SCR) emission control system.

• T8.275
• T8.320
• T8.350
• T8.380
• T8.410
• T8.435
• CR6.80
• CR6.90
• CR7.90
• CR8.90
• CR9.90
• CR10.90

1. 1

   Check the ECM for any other active or stored fault codes related to NOx sensors, temperature sensors, DEF (AdBlue) quality, or the SCR system

2. 2

   Inspect wiring harnesses and connectors for all after-treatment components (NOx sensors, temperature sensors, DEF injector) for corrosion, chafing, heat damage, or loose pins

3. 3

   Monitor live data from sensors that influence the NOx estimation model (intake manifold temperature, ambient air temperature/humidity, and exhaust temperature) for unreliable readings

4. 4

   Inspect the SCR system for physical issues, such as exhaust leaks before and around the SCR catalyst, or DEF injector crystallization/blockage

5. 5

   Test the quality and concentration of the Diesel Exhaust Fluid (DEF)

1. 1

   Address other active faults

   Resolve any other active diagnostic trouble codes first, especially those related to specific sensors or the DEF system. Clearing these faults may resolve the 3521 code.

2. 2

   Perform system reset and re-calibration

   After addressing related faults, clear all codes and perform a test run. Use an OEM service tool to perform any required service routines or re-calibration procedures after component replacement.

3. 3

   Check NOx sensor plausibility

   If no other codes are present, monitor live NOx readings from upstream and downstream sensors on a warmed-up engine under load. The upstream sensor should show significantly higher NOx ppm values. If a sensor reading is frozen (e.g., at 0 ppm), erratic, or does not change with engine load, replace the faulty sensor.

4. 4

   Update ECM software

   Check with a certified dealer for any available software updates for the Engine Control Module that may improve the logic of diagnostic monitors and estimation models.

5. 5

   Replace ECM

   As a last resort, if all sensors, wiring, and the SCR system have been verified to be in good working order, replace the ECM.