Massey Ferguson 521012.31 – Engine ECU: NOx Sensor Outlet SCR Signal Issue

The controller has detected an implausible, stuck, or erratic peak value from the downstream (outlet) NOx sensor located after the Selective Catalytic Reduction (SCR) catalyst. The sensor is either failing to stabilize, reporting an out-of-range measurement, or suffering from a communication/circuit 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
• 4700 Series
• 5700 Series
• 5708
• 5709
• 5710
• 5711
• 5712
• 5713
• 6700 Series
• 6711
• 6712
• 6713
• 6714
• 6715
• 6716
• 6718
• 7700 Series
• 7714 through 7726
• 8700 Series
• 8727 through 8740
• 7300 Combine Series
• Activa
• BETA

1. 1

   Inspect the downstream NOx sensor wiring harness for heat soak damage, vibration chafing, or melted wires. Ensure the connector pins are free of corrosion and moisture.

2. 2

   Ensure the DEF tank is at least one-third full and the fluid is uncontaminated (ISO 22241-compliant). Poor DEF quality can cause the SCR system to underperform, triggering downstream NOx plausibility codes.

3. 3

   Inspect the exhaust piping between the DOC/SCR and the outlet NOx sensor for leaks. Air intrusion can skew the sensor's readings.

4. 4

   Verify that the sensor is receiving proper supply voltage from the ECU.

1. 1

   Preparation

   Turn off the tractor and allow the exhaust system to cool completely to avoid burns.

2. 2

   Disconnect

   Locate the downstream NOx sensor (at the exhaust outlet, post-SCR). Disconnect the NOx sensor module's electrical connector from the main engine harness.

3. 3

   Removal

   Using a specialized NOx sensor socket or a standard wrench (usually 22mm), carefully unscrew the sensor from the exhaust bung.

4. 4

   Installation

   Apply a small amount of anti-seize to the threads of the new sensor (if not pre-applied by the manufacturer), being extremely careful not to get any on the sensor tip. Thread the new sensor into the exhaust outlet and tighten to OEM torque specifications.

5. 5

   Routing

   Reconnect the electrical connector. Crucial step: Route the new NOx sensor harness exactly like the factory routing. Improper routing is a leading cause of premature failure due to heat soak and vibration.

6. 6

   Calibration/Clear Codes

   Use a diagnostic tool (such as AGCO EDT) to clear the fault codes. Run the engine through a warm-up cycle; the ECU requires a specific amount of time to stabilize and calibrate the new NOx sensor readings.