Deutz-Fahr 128 – Engine ECU: Upstream NOx Sensor CAN Timeout

The Engine Control Module (ECM/EMR) has lost communication with the upstream NOx sensor located before the catalyst. The system detected a timeout in the reception of a J1939 CAN message, indicating that the expected data packets from the sensor are no longer detected on the network.

• Agrotron 128

1. 1

   Visually inspect the wiring harness and connectors between the Engine ECU and the upstream NOx sensor for chafing, melting from exhaust contact, corrosion, or bent/backed-out pins

2. 2

   Examine the NOx sensor for heavy soot, oil, or coolant contamination, which can indicate other engine issues causing sensor failure

3. 3

   Perform CAN bus physical layer tests with the system powered off: measure resistance across CAN High and CAN Low (should be approximately 60 Ω; 120 Ω indicates a missing termination resistor or broken line)

4. 4

   Check CAN bus voltage with ignition on (engine off): CAN High should be 2.5-3.0V and CAN Low should be 2.0-2.5V

5. 5

   Check for short circuits by measuring resistance from CAN High/Low to ground (should read in megaohms)

6. 6

   Verify the NOx sensor module is receiving the correct power supply voltage and has a solid ground connection

1. 1

   Address Wiring Issues

   If any faults were found during the diagnostic checks, repair them first. This includes fixing broken wires, cleaning corroded connectors, or securing loose connections. After repairs, clear the fault codes and re-test.

2. 2

   Isolate the Sensor

   If the wiring and CAN bus network test healthy, the fault is likely with the NOx sensor module itself. The sensor consists of the probe that sits in the exhaust and an integrated electronic module that communicates on the CAN bus.

3. 3

   Replace the NOx Sensor

   If the sensor is determined to be faulty, it must be replaced. Disconnect the electrical connector, carefully unscrew the NOx sensor probe from the exhaust pipe (pre-soaking the threads with penetrating oil is recommended), install the new sensor ensuring the correct torque is applied, and reconnect the electrical connector.

4. 4

   Clear Adaptations and Re-Test

   After replacing the sensor, use a diagnostic tool to clear fault codes and reset the SCR system's learned adaptations or variant coding. Start the engine and verify that the fault code does not return.