Massey Ferguson 521017.31 – Engine ECU: NOx Sensor Before DOC Signal Issue

The engine control system is receiving an implausible, stuck, or missing signal from the upstream Nitrogen Oxide (NOx) sensor.

• 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
• 5608
• 5609
• 5610
• 5611
• 5612
• 5613
• 5710 SL
• 5711 SL
• 5712 SL
• 5713 SL
• 7614-7626
• 7714-7726S

1. 1

   Inspect the wiring harness and connector leading to the upstream NOx sensor for chafed wires, melted insulation, disconnected plug, water intrusion, or corrosion.

2. 2

   Inspect the exhaust manifold and piping before the DOC for leaks, which can introduce outside air and skew NOx sensor readings.

3. 3

   Remove the sensor and inspect the tip for heavy soot or carbon buildup that could blind the sensor.

4. 4

   Verify that the sensor is receiving proper supply voltage from the ECU and that the internal heater circuit is not shorted or open.

1. 1

   Preparation

   Turn off the tractor, remove the key, and disconnect the negative battery cable. Allow the exhaust system to cool completely.

2. 2

   Locate the Sensor

   Locate the Upstream NOx sensor. It is threaded into the exhaust pipe directly before the DOC (Diesel Oxidation Catalyst) housing.

3. 3

   Disconnect

   Unplug the electrical connector for the NOx sensor from the main engine wiring harness. Free the wire from any retaining clips.

4. 4

   Removal

   Using a specialized NOx/O2 sensor socket or a suitable wrench, carefully unscrew the sensor from the exhaust pipe. Note: If the sensor is seized, apply penetrating oil to the threads and allow it to soak to prevent stripping the exhaust bung.

5. 5

   Installation

   Apply a small amount of high-temperature anti-seize compound to the threads of the new sensor (if it does not come pre-applied from the factory). Thread the new sensor into the exhaust pipe by hand to prevent cross-threading, then tighten to the manufacturer's specified torque.

6. 6

   Reconnect

   Route the sensor wiring exactly as it was originally, ensuring it is secured away from extreme heat sources. Plug the connector back into the harness.

7. 7

   Clear Codes

   Reconnect the battery. Connect a diagnostic scan tool to the tractor, clear the inactive 521017.31 fault code from the Engine ECU, and run the engine to operating temperature to verify the code does not return.