Deutz-Fahr 659 – ECM: Erratic Downstream NOx Sensor Signal

The Engine Control Module (ECM) has detected that the signal from the downstream NOx sensor (located after the SCR catalyst) is erratic, intermittent, or incorrect. This sensor measures nitrogen oxide concentrations to verify SCR system efficiency. An unstable or implausible signal indicates a potential sensor failure, an exhaust leak introducing oxygen, or a wiring circuit issue.

• TCD 3.6 L4
• TCD 3.6 T4F

1. 1

   Retrieve all fault codes to check for related SCR or DEF system faults that could provide additional context

2. 2

   Inspect the aftertreatment system and exhaust pipes near the downstream NOx sensor for exhaust leaks, which can introduce oxygen and skew readings

3. 3

   Examine the downstream NOx sensor wiring harness and connectors for corrosion, chafing, heat damage, or loose pins. Perform a wiggle test while monitoring live data

4. 4

   Compare live data from upstream and downstream NOx sensors at operating temperature. The downstream reading should be significantly lower and stable; rapid fluctuations indicate a sensor or circuit issue

5. 5

   Test DEF quality using a refractometer (should be approximately 32.5% urea) and inspect the DEF dosing valve for crystallization

6. 6

   Verify the NOx sensor is receiving correct voltage and has a solid ground connection according to specifications

1. 1

   Repair preliminary faults

   Fix any identified exhaust leaks, wiring damage, or DEF system issues found during initial checks. Clear fault codes and test operate the machine to see if the fault returns.

2. 2

   Clean NOx sensor

   If the sensor is heavily coated in soot, carefully clean it. If the sensor is failing internally, proceed to replacement.

3. 3

   Remove faulty NOx sensor

   Allow the exhaust system to cool completely. Disconnect the electrical connector and remove the sensor module. Apply penetrating oil to the probe threads and carefully remove the probe from the exhaust bung using a slotted socket.

4. 4

   Prepare exhaust bung

   Clean the threads in the exhaust bung using an M20 x 1.5 thread chaser to ensure the new sensor can be installed without damage.

5. 5

   Install new NOx sensor

   Apply a small amount of ceramic-based anti-seize compound to the threads of the new sensor probe (avoiding the sensor tip). Install the probe, torque to manufacturer specification, mount the sensor module, and securely connect the electrical harness.

6. 6

   Perform post-repair calibration

   Clear all fault codes using a diagnostic tool. Perform a system reset or calibration for the new NOx sensor if required, then run a parked regeneration to verify that SCR system efficiency is restored.