Case IH 3521 – Engine Control Unit (ECU) / Engine Control Module (ECM): NOx Estimation failure - Estimated Nox signal not reliable

The Engine Control Module (ECM) continuously compares the INLET (Pre-SCR) and OUTLET (Post-SCR) NOx sensor readings to evaluate the efficiency of the Selective Catalytic Reduction (SCR) system. Code 3521 triggers when the ECM determines that the estimated NOx signal is unreliable or insufficient. This typically points to a faulty NOx sensor providing inaccurate readings, wiring issues, or excessive Diesel Exhaust Fluid (DEF) / soot buildup on the sensor probe.

• Magnum (180 to 380)
• Steiger (350 to 620)
• Quadtrac (370 to 620)
• Puma (150 to 220)
• Maxxum (115 to 150)
• Farmall (110A to 140A)
• Axial-Flow 5130
• Axial-Flow 6130
• Axial-Flow 7130
• Axial-Flow 7230
• Axial-Flow 8230
• Axial-Flow 9230
• Titan 2240
• Titan 3030
• Titan 3530
• Titan 4030
• Titan 4530
• Case CE Loaders (521F through 1121F)

1. 1

   Verify Active Codes: Use an engine diagnostic tool (like CNH EST) to verify if the code is active. Check if there are other active codes related to the SCR system or DEF quality (e.g., 3364), which might indicate a broader DEF system issue.

2. 2

   Operating Temperature: Ensure the exhaust is up to operating temperature and free of moisture. NOx sensors are not operational until the exhaust reaches a specific temperature threshold.

3. 3

   Wiring and Connectors: Inspect the NOx sensor circuit for damaged or shorted wiring. Check the sensor connectors for looseness, corrosion, or water ingress.

4. 4

   Voltage Test: Test the voltage and resistance from the sensor to ensure it is receiving a stable power supply and is within OEM specifications.

5. 5

   Visual Probe Inspection: Remove the NOx sensor and inspect the probe for excessive DEF crystallization or soot buildup, which can blind the sensor and cause unreliable readings.

1. 1

   Remove the affected NOx sensor

   Remove the affected NOx sensor from the exhaust pipe.

2. 2

   Clean the sensor probe

   Carefully clean DEF or soot buildup from the probe area using a soft wire brush. Caution: Avoid using harsh chemicals, solvents, or abrasive materials that could damage the sensing element. Do not allow the sensor tip to become wet.

3. 3

   Reinstall and test

   Reinstall the sensor, clear the codes, and run the machine to operating temperature to see if the code returns.

4. 4

   Power down the machine

   If the sensor has failed, power down the machine and disconnect the battery to prevent electrical shorts.

5. 5

   Disconnect the NOx sensor

   Disconnect the NOx sensor electrical connector.

6. 6

   Remove the faulty NOx sensor

   Unthread and remove the faulty NOx sensor from the exhaust pipe. Note: Do NOT cut the sensor wiring harness, as failed NOx sensors are often required for core returns or warranty claims.

7. 7

   Install the new NOx sensor

   Install the new replacement NOx sensor and tighten it to the manufacturer's specified torque.

8. 8

   Reconnect the electrical connector

   Reconnect the electrical connector.

9. 9

   Reconnect the battery and clear codes

   Reconnect the battery and use a diagnostic tool to clear the stored DTCs. Error clearing is mandatory after fitting a new NOx sensor to reset the ECM's estimation logic.