Massey Ferguson 521018.31 – Engine ECU: SCR NOx Levels Too High (ECU)

This fault code indicates that the Selective Catalytic Reduction (SCR) system is failing to reduce Nitrogen Oxide (NOx) emissions to acceptable levels. The downstream NOx sensor is detecting higher-than-expected NOx concentrations exiting the tailpipe compared to the upstream sensor's readings. This typically triggers an engine derate (limp mode) to protect the environment and comply with emissions regulations.

• 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
• 5709
• 5710
• 5711
• 5713
• 7717
• 7721
• 7723
• 7725

1. 1

   DEF / AdBlue Quality Check: Contaminated fluid (dirt, water, or diesel) is a leading cause of SCR inefficiency. Check the DEF tank for debris and test the fluid concentration (it should be exactly 32.5% urea).

2. 2

   Live Data Monitoring: Use a diagnostic tool to monitor the upstream (pre-catalyst) and downstream (post-catalyst) NOx sensors. If the downstream sensor's readings do not drop under load, or if the sensor is stuck at a static value, the sensor is likely drifting or failing.

3. 3

   Inspect the DEF Dosing Module (Injector): Remove the DEF injector from the exhaust pipe and inspect it for severe urea crystallization, clogging, or fouling.

4. 4

   Check DEF Pump and Lines: Ensure the DEF pump is building adequate pressure and that there are no kinks, leaks, or damaged O-rings in the heated supply lines.

1. 1

   Resolve Fluid Issues

   If the DEF is contaminated or degraded, drain the tank completely, flush the system, replace the DEF pump filter/strainer, and refill with fresh AUS-32 DEF.

2. 2

   Clean or Replace the Dosing Module

   If the DEF injector is caked in crystallized urea, clean it using warm distilled water. If it is seized or electronically failed, replace the dosing module.

3. 3

   Replace Faulty NOx Sensor(s)

   If live data confirms a sensor is reading out of range (often caused by heat degradation or soot contamination), replace the affected NOx sensor.

4. 4

   System Reset and Adaptation

   After replacing the NOx sensor or dosing module, you must use a compatible diagnostic tool (e.g., dealer tool or advanced aftermarket tool like TOPDON Phoenix Elite) to clear the fault codes and perform an SCR system reset/NOx sensor adaptation. The code will not clear on its own just by replacing the part.