Massey Ferguson 521025.31 – Engine Control Unit (ECU): SCR NOx Levels Too High (ECU)

The Selective Catalytic Reduction (SCR) system is failing to efficiently convert nitrogen oxides (NOx) into harmless nitrogen and water. The downstream NOx sensor is reading emission levels that exceed the allowable threshold set by Tier 4 / Stage V emissions standards.

• 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
• MF 4707
• MF 4708
• MF 4709
• MF 4710

1. 1

   Test DEF/AdBlue concentration using a refractometer (should be 32.5% urea) to check for poor-quality, degraded, or contaminated fluid.

2. 2

   Inspect the DEF dosing injector for crystallized DEF deposits that can block the nozzle and reduce fluid injection.

3. 3

   Inspect the wiring, connectors, and pins on both the upstream and downstream NOx sensors for corrosion, water ingress, or physical damage.

4. 4

   Verify that the DEF pump/supply module is building adequate pressure to supply the injector.

5. 5

   Check the DEF filters for clogging, which can restrict fluid flow.

1. 1

   Address DEF Fluid

   If the DEF is contaminated or out of spec, drain the DEF tank completely, flush it, and refill it with fresh, high-quality DEF.

2. 2

   Clean or Replace the DEF Injector

   Remove the DEF injector from the exhaust pipe. If heavy crystallization is present, clean it using warm distilled water. If it is mechanically failed, replace it.

3. 3

   Replace DEF Filters

   Replace the DEF supply pump filters as part of standard maintenance to ensure proper fluid pressure.

4. 4

   Replace the NOx Sensor

   If the DEF system is dosing correctly but the code persists (or if the sensor fails electrical testing), replace the downstream NOx sensor.

5. 5

   Clear Codes and Test

   Use an OEM diagnostic tool to clear the fault code. Initiate a forced active regeneration or an SCR system dosing test to verify that NOx conversion levels have returned to normal.