Deutz-Fahr 138 – ECU: Post-Cat NOx Sensor Message Length Error

The fault code indicates a CAN bus data communication error between the post-catalyst (downstream) NOx sensor and the Engine Control Unit (ECU). The ECU is receiving data packets of an incorrect or unexpected length, preventing it from correctly interpreting the sensor's readings and monitoring the efficiency of the Selective Catalytic Reduction (SCR) system.

• 5 Series
• 6 Series
• 7 Series
• 8 Series
• 9 Series
• Deutz TCD 2013
• Deutz TCD 2015
• Deutz TCD 2016
• Deutz TCD 2020
• Deutz TCD 2021
• Deutz TCD 3.6
• Deutz TCD 4.1

1. 1

   Visually inspect the entire wiring harness between the post-catalyst NOx sensor and the engine ECU for chafing, melting from exhaust contact, or pinched wires

2. 2

   Disconnect and inspect connectors at the NOx sensor and ECU for corrosion (green or white powder), moisture ingress, or bent/backed-out pins

3. 3

   With the key on and sensor disconnected, use a multimeter to verify the sensor is receiving correct voltage and has a solid ground connection at the harness connector

4. 4

   Test the CAN communication wires (CAN-H and CAN-L) at the sensor connector with key on and engine off (expect approximately 2.6V on CAN-H and 2.4V on CAN-L)

5. 5

   Clear all related fault codes from the ECU after performing inspections and wiring repairs to see if the fault returns

1. 1

   Isolate the sensor

   If the pre-catalyst and post-catalyst NOx sensors have identical part numbers, swap them to see if the fault code follows the sensor. Note: Many NOx sensors have unique CAN IDs and cannot be swapped.

2. 2

   Replace the post-catalyst NOx sensor

   If wiring is sound or the fault followed the sensor swap, replace the sensor. Disconnect the battery, unplug the electrical connector, and apply penetrating oil to unscrew the sensor probe from the exhaust bung. Install the new sensor probe to the manufacturer's torque specification, mount the module, connect the harness, and reconnect the battery.

3. 3

   Perform ECU reset/relearn

   Use a diagnostic tool to perform an ECU reset or a specific NOx sensor relearn procedure. This clears adaptation data and ensures the ECU communicates correctly with the new sensor.