Massey Ferguson 3235.03 – Engine Control Unit (ECU): NOx Sensor Outlet SCR Signal Issue. (ECU)

This indicates a signal performance problem—such as an open circuit, short circuit, or out-of-range voltage—with the downstream (outlet) Nitrogen Oxide (NOx) sensor. This sensor is located after the Selective Catalytic Reduction (SCR) catalyst and is responsible for measuring the amount of NOx exiting the tailpipe to ensure the SCR system is effectively reducing emissions.

• 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
• 7614
• 7615
• 7616
• 7618
• 7619
• 7620
• 7622
• 7624
• 7626
• 7714S
• 7719S

1. 1

   Inspect the wiring harness and connector for the SCR Outlet NOx sensor for corrosion, pin-back, or wires chafing against hot exhaust components

2. 2

   Check the signal wire resistance (a resistance above 0.5 ohms indicates a wiring fault)

3. 3

   Measure the resistance across the heater pins with the sensor disconnected and cold (should typically be between 2 and 6 ohms)

4. 4

   Use a diagnostic tool to monitor live NOx ppm values; if the reading is frozen at 0 ppm or maxed out while the engine is running and warmed up, the sensor element has failed

5. 5

   Ensure the DEF fluid is not contaminated and that the SCR catalyst is not saturated with ammonia (ammonia slip), which can trick the sensor into reading high NOx levels

1. 1

   Preparation

   Turn off the engine, apply the parking brake, and disconnect the batteries to prevent electrical shock. Ensure the exhaust system has cooled down.

2. 2

   Removal

   Locate the SCR Outlet NOx sensor downstream of the SCR catalyst. Disconnect the electrical connector from the ECU/harness. Cut any zip-ties securing the sensor's wiring harness.

3. 3

   Extract the Sensor

   Use a specialized slotted O2/NOx sensor socket to carefully unscrew the sensor from the exhaust pipe.

4. 4

   Preparation of New Sensor

   Apply a small amount of anti-seize compound to the threads of the new NOx sensor (many new OEM sensors come with this pre-applied). Caution: Do not over-apply, as excess anti-seize can contaminate the sensor tip.

5. 5

   Installation

   Thread the new sensor into the exhaust pipe and torque it to the OEM specification (typically around 50 N·m or 37 lb·ft).

6. 6

   Reconnection

   Reconnect the electrical connector. Secure the wiring harness with zip-ties, ensuring it is routed exactly like the factory setup to avoid heat soak and vibration damage.

7. 7

   Calibration

   Reconnect the batteries. Use a diagnostic tool to clear the fault codes. Note: Many modern Euro 6 / Tier 4 systems require variant coding or calibration of the new NOx sensor via the diagnostic tool; skipping this may cause the fault code to persist.