Massey Ferguson U 520308 – Engine ECU or DCU: NOx sensor plausibility error: Signal not plausible.

The control unit is receiving a reading from the Nitrogen Oxide (NOx) sensor that is outside the expected or logical range for the current engine operating conditions, indicating a potential failure in the sensor's ability to accurately measure NOx concentrations in the exhaust gas.

• 8600 Series (8650, 8660, 8670, 8680, 8690)
• 5600 Series (5611-5613)
• 6600 Series (6612-6616)
• 7600 Series (7614-7626)
• 8700 Series (8727-8740)
• 5700 SL Series
• 6700 S Series

1. 1

   Inspect the NOx sensor wiring harness and connectors for chafing, melting, or loose pins. Ensure the harness is routed correctly away from extreme heat sources.

2. 2

   Ensure the DEF tank is at least one-third full and that the fluid is ISO 22241-compliant and uncontaminated. Poor DEF quality can cause poor NOx conversion, triggering plausibility errors.

3. 3

   Check for exhaust leaks upstream of the NOx sensor, which can introduce unmetered oxygen and skew the sensor's readings.

4. 4

   Check the sensor for mechanical damage, heavy soot/carbon buildup, or moisture intrusion.

1. 1

   Preparation

   Disconnect the cable from the negative battery terminal. Ensure the exhaust system has completely cooled to avoid burns.

2. 2

   Access

   Locate the NOx sensor on the exhaust/SCR system. If necessary, remove any exhaust heat shields retained by bolts.

3. 3

   Removal

   Disconnect the electrical connector. Use a slotted oxygen/NOx sensor socket to remove the sensor. Tip: If the sensor is seized, spray penetrating oil onto the threads and allow it to soak before applying force.

4. 4

   Preparation for New Sensor

   Ensure the exhaust pipe threads are clean and free of heavy carbon deposits.

5. 5

   Installation

   Install the new NOx sensor. Crucial: Do not touch the ceramic probe with bare fingers, as oil contamination will cause premature failure. Do not use an impact wrench to install the sensor, as this can damage the threads and internal ceramics.

6. 6

   Routing

   Route the NOx sensor harness exactly like the factory routing. This is a known best practice to avoid heat soak and vibration failures.

7. 7

   Completion

   Reconnect the electrical connector and the negative battery terminal. Clear the OBD fault codes using a diagnostic tool and perform a functional test/power activation to verify the repair.