Massey Ferguson 521019.31 – Engine ECU: SCR NOx Levels Too High (ECU) / SCR Efficiency Threshold 2 Exceeded

This code indicates that the Selective Catalytic Reduction (SCR) system is failing to reduce Nitrogen Oxide (NOx) emissions below the required regulatory threshold. The downstream NOx sensor is detecting higher-than-expected NOx levels in the exhaust gas compared to the upstream sensor, meaning the catalytic reduction process is inefficient.

• 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
• 5708
• 5709
• 5710
• 5711
• 5712
• 5713
• 6711
• 6712
• 6713
• 6714
• 6715
• 6716
• 6718
• 7614
• 7615
• 7616
• 7618
• 7619
• 7620
• 7622
• 7624
• 7626
• 8727
• 8730
• 8732
• 8735
• 8737
• 8740

1. 1

   Check DEF/AdBlue fluid quality for contamination or crystallization (bad, diluted, or expired DEF is a primary cause)

2. 2

   Inspect the DEF dosing system, including the DEF injector for blockages or crystallized urea buildup, and ensure the DEF pump delivers adequate pressure

3. 3

   Inspect both upstream and downstream NOx sensors for physical damage, heavy soot accumulation, or chemical buildup on the sensor tips

4. 4

   Check wiring harnesses and connectors leading to the NOx sensors and DEF injector for chafing, corrosion, or water ingress

5. 5

   Check for accompanying fault codes (e.g., 521022, 521023, 521024, or 521007) that might indicate a specific sensor failure, pressure stabilization issue, or interrupted dosing

1. 1

   Address Fluid Issues

   If the DEF fluid is suspected to be old or contaminated, drain the DEF tank, flush the system, and refill it with fresh, high-quality DEF.

2. 2

   Clean or Replace the DEF Injector

   Remove the DEF dosing module from the exhaust pipe. If there is crystallized urea, clean it carefully with warm water. If the injector is mechanically failed or severely clogged, replace it.

3. 3

   Replace Faulty NOx Sensors

   If the DEF system is dosing correctly but the code persists (or if the sensors are heavily coated in soot/damaged), replace the faulty NOx sensor(s).

4. 4

   Clear Codes and Test

   Use a diagnostic tool to clear the fault codes from the Engine ECU. Perform a forced SCR system test or operate the tractor under a heavy load to ensure exhaust temperatures rise enough for the SCR system to function, verifying that the efficiency returns to normal and the code does not return.