Massey Ferguson 521013.31 – Engine ECU: NOx Sensor Outlet SCR Signal Issue / Downstream NOx sensor value stuck.

This code indicates that the signal from the downstream NOx sensor (located at the outlet of the Selective Catalytic Reduction [SCR] catalyst) is "stuck" or implausible. The Engine Control Unit is not seeing the expected fluctuations in nitrogen oxide levels after the exhaust gases pass through the SCR system, meaning the system cannot accurately verify if the DEF (AdBlue) dosing is effectively reducing emissions.

• 4710
• 7724S
• 5712SL
• 7722S
• 7722
• 5713SL
• 7720S
• 7716
• 7718
• T4
• 7715
• 5710SL
• 7726
• 4708
• 7726S
• 4707
• 4709
• 5711SL
• 7718S
• 7719
• 7714
• 7716S
• T4 (2015-2021)
• 7720
• Tier 4 (2015-2021)
• 7715S
• 7724
• T4 Models (2015-2021)

1. 1

   Wiring & Connector Inspection: Check the downstream NOx sensor wiring harness and control module connector for melted wires, chafing, or water ingress.

2. 2

   DEF (AdBlue) Quality & Dosing: Verify that the DEF tank has high-quality, uncontaminated fluid. Check the DEF injector and pump for blockages or pressure issues. If the system is not injecting DEF properly, the downstream NOx levels will not change, causing the ECU to think the sensor is stuck.

3. 3

   Exhaust Leaks: Inspect the exhaust pipe for leaks anywhere between the engine turbocharger and the SCR catalyst.

4. 4

   Physical Sensor Inspection: Remove the sensor and inspect the probe. Heavy soot packing or white DEF crystallization can blind the sensor, causing a stuck signal.

1. 1

   Power Down

   Turn off the tractor and disconnect the batteries, starting with the negative (-) terminal, to prevent electrical shorts.

2. 2

   Locate the Sensor

   Locate the downstream NOx sensor, which is mounted on the exhaust pipe after the SCR catalyst.

3. 3

   Disconnect Wiring

   Unplug the main electrical connector for the NOx sensor and disengage the wiring harness from any retaining clips.

4. 4

   Remove the Control Unit

   Remove the mounting nuts securing this control unit to the chassis/bracket.

5. 5

   Remove the Sensor Probe

   Using a specialized NOx sensor socket or wrench, carefully unscrew the sensor probe from the exhaust pipe.

6. 6

   Install the New Sensor

   Apply a small amount of high-temperature anti-seize to the threads of the new sensor (if not already pre-applied by the manufacturer). Thread it into the exhaust pipe and tighten to OEM torque specs.

7. 7

   Mount and Route

   Secure the new sensor's control unit to the mounting bracket with the nuts. Route the wiring harness safely away from extreme heat sources and plug in the connector.

8. 8

   Reconnect Power

   Reconnect the battery terminals (positive first, then negative).

9. 9

   Software Reset

   Connect the AGCO Electronic Diagnostic Tool (EDT). Clear the inactive 521013.31 fault code and perform any required SCR system resets or NOx sensor initialization procedures.