Massey Ferguson 4090.31 – Engine Control Unit (ECU): NOx Are High In The Exhaust

The engine control module has detected that the nitrogen oxide (NOx) levels in the exhaust stream exiting the Selective Catalytic Reduction (SCR) system are higher than the permitted emissions threshold. This typically indicates that the SCR system is not effectively neutralizing NOx, which can be caused by a faulty NOx sensor, poor DEF (Diesel Exhaust Fluid) quality, or a DEF dosing issue.

• 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
• 7615
• 7616
• 7618
• 7619
• 7620
• 7622
• 7624
• 7626

1. 1

   Verify DEF Quality and Level: Ensure the DEF tank is at least one-third full and that the fluid is ISO 22241-compliant. Contaminated or degraded DEF (e.g., mixed with diesel or water) will fail to reduce NOx, triggering this code.

2. 2

   Inspect DEF Lines and Doser: Check for leaks, damaged O-rings, or heavy white crystallization around the DEF injector/dosing valve. A clogged doser will prevent DEF from entering the exhaust stream.

3. 3

   Check NOx Sensor Wiring: Inspect the NOx sensor wiring harness for heat soak damage, chafing, or vibration-induced breaks. Ensure the connector pins are clean and securely seated.

4. 4

   Check for Exhaust Leaks: Inspect the exhaust piping upstream of the SCR catalyst and NOx sensor. Exhaust leaks can introduce excess oxygen or alter pressure, skewing the sensor's readings.

1. 1

   Preparation

   Park the tractor on a level surface, turn off the engine, and allow the exhaust system to cool completely to avoid severe burns.

2. 2

   Disconnect the Harness

   Locate the faulty NOx sensor on the exhaust pipe. Carefully unplug the sensor's electrical connector from the main engine harness.

3. 3

   Remove the Sensor

   Using a specialized NOx sensor socket or a properly sized wrench, unscrew the sensor from the exhaust pipe. (Penetrating oil may be required if the threads are seized from heat cycling).

4. 4

   Prepare the New Sensor

   Apply a small amount of high-temperature anti-seize compound to the threads of the new NOx sensor. Caution: Do not get any anti-seize on the sensor tip, as this will permanently contaminate it.

5. 5

   Installation

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

6. 6

   Route the Wiring

   Reconnect the electrical harness. It is critical to route the new harness exactly like the factory routing to prevent future heat soak or vibration failures.

7. 7

   Clear Codes

   Connect a diagnostic scan tool to the tractor and clear the stored 004090.31 fault code to reset the SCR system and bring the tractor out of any active derate modes.