Massey Ferguson U 520228 – Engine Control Unit (ECU) / EEM4 / DCU: NOx emission control error: NOx threshold exceeded

The NOx sensor located downstream of the SCR catalyst is detecting emissions levels that are higher than the permitted environmental threshold. The system is failing to properly reduce the exhaust gases, which will often force the tractor into a derate (reduced power) mode.

• 5600 Series
• 6600 Series
• 7600 Series
• 8600 Series
• 8650
• 8660
• 8670
• 8680
• 8690
• 5700 Series
• 6700 Series
• 7700 Series
• 8700 Series
• MF 9540 Combine
• MF 9560 Combine

1. 1

   Check DEF/Urea quality and level — poor quality, diluted, or contaminated DEF will fail to reduce NOx (look for codes U 520227 or U 520229)

2. 2

   Check for other SCR codes related to DEF pressure, DEF temperature, or dosing interruptions (e.g., U 520226)

3. 3

   Inspect the DEF dosing valve (injector) to ensure it is not clogged with crystallized urea and is spraying correctly

4. 4

   Check for exhaust leaks upstream of the SCR catalyst, which can introduce excess oxygen or allow exhaust to escape before treatment

5. 5

   Inspect the NOx sensor wiring harness and connectors for melted wires, chafing, or corroded pins

1. 1

   Cool Down

   Ensure the tractor is turned off and the exhaust system has completely cooled down to prevent severe burns.

2. 2

   Disconnect Power

   Disconnect the negative (-) battery terminal to prevent electrical shorts and protect the ECU.

3. 3

   Locate the Sensor

   Locate the downstream NOx sensor threaded into the exhaust pipe after the SCR catalyst unit.

4. 4

   Disconnect Wiring

   Carefully unplug the NOx sensor's electrical connector from the main engine wiring harness.

5. 5

   Remove the Sensor

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

6. 6

   Clean the Port

   Clean the mounting threads in the exhaust pipe to remove soot and rust.

7. 7

   Prepare the New Sensor

   Apply a high-temperature anti-seize compound to the threads of the new NOx sensor (if it does not already come pre-applied from the factory). Do not get anti-seize on the sensor probe itself.

8. 8

   Install the New Sensor

   Thread the new NOx sensor into the exhaust pipe by hand to prevent cross-threading, then tighten to the OEM specified torque.

9. 9

   Reconnect Wiring

   Plug the new sensor's electrical connector back into the wiring harness, ensuring the locking tab clicks into place.

10. 10

    Restore Power

    Reconnect the negative battery terminal.

11. 11

    Clear Codes and Reset

    Connect a diagnostic tool (like AGCO EDT) to the tractor. Clear the stored U 520228 fault code. You may also need to perform an SCR system reset or NOx sensor adaptation routine so the ECU recognizes the new sensor.

12. 12

    Test Run

    Start the engine and allow it to reach operating temperature. Verify that the fault code does not return and the tractor exits derate mode.