Massey Ferguson 521020.31 – Engine ECU: SCR NOx Levels Too High (ECU) / SCR efficiency threshold 3 exceeded

The Selective Catalytic Reduction (SCR) system is failing to reduce Nitrogen Oxide (NOx) emissions to the required legal limits. The Engine ECU monitors the exhaust gases using upstream and downstream NOx sensors. When the downstream sensor detects that NOx levels have exceeded "Threshold 3" (a critical degradation state), it triggers this code. This typically points to poor Diesel Exhaust Fluid (DEF/AdBlue) quality, a failing DEF dosing system, faulty NOx sensors, or a degraded SCR catalyst.

• 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
• 5711
• 5712
• 5713
• 6711
• 7717
• 7721
• 7723
• 7725
• 8670
• 8690
• 8727
• 8730
• 8732

1. 1

   Check for Accompanying Codes: Look for related codes that might pinpoint the root cause, such as DEF pressure faults (e.g., 521007.10 / 521007.14) or specific NOx sensor circuit errors (e.g., 521012.31, 521023.31).

2. 2

   Test DEF / AdBlue Quality: Contaminated, diluted, or expired DEF is the #1 cause of SCR efficiency codes. Use a DEF refractometer to ensure the fluid is exactly 32.5% urea.

3. 3

   Inspect the DEF Dosing System: Check the DEF injector/dosing valve for crystallization, blockages, or soot buildup. Ensure the DEF pump is building adequate pressure.

4. 4

   Inspect Exhaust System: Check for exhaust leaks upstream of the SCR catalyst. A leak can alter the exhaust flow and temperature, preventing the DEF from converting to ammonia properly.

5. 5

   Check NOx Sensors: Inspect the wiring harnesses for the upstream and downstream NOx sensors for chafing, melting, or poor connections.

1. 1

   Address Fluid Quality

   If the DEF is degraded or contaminated, drain the DEF tank completely. Flush the system and replace the main DEF pump filter. Refill with fresh, high-quality DEF.

2. 2

   Clean the Doser

   Remove the DEF dosing valve from the exhaust pipe. Clean any crystallized urea or soot using warm distilled water. Reinstall with a new gasket/O-ring.

3. 3

   Run a System Test / Forced Regen

   Connect an AGCO diagnostic tool (EDT) and perform an SCR system test or a forced stationary regeneration. This will test the dosing quantity and monitor the NOx sensor readings in real-time.

4. 4

   Replace NOx Sensors

   If the diagnostic tool shows the downstream NOx sensor reading erratically, stuck, or mirroring the upstream sensor despite proper DEF dosing, replace the NOx sensor(s).

5. 5

   Replace SCR Catalyst (Worst Case)

   If DEF quality is perfect, the doser is spraying correctly, and the NOx sensors are functioning but the efficiency is still below the threshold, the SCR catalyst itself has been poisoned or degraded and must be replaced.