Case IH 19543 – Engine Control Unit (ECU): NOx offset max error detection

The Engine Control Unit (ECU) has detected that the downstream NOx sensor (located post-SCR) is reading a calibration offset value that exceeds the maximum allowable threshold. This indicates that the sensor is either providing implausible readings, has an internal circuit failure, or is being skewed by an external factor like a wiring issue or exhaust leak.

• Tier 4b
• Case N Series Loaders
• Case IH tractors
• Case Construction equipment

1. 1

   Check for damaged or loose machine grounds and connector pins, looking for green crust, debris, or water inside the connector terminals

2. 2

   Inspect the wiring harness for physical damage, heat damage near the exhaust routing, or incorrect routing

3. 3

   Check the exhaust tract for leaks upstream of the sensor, as exhaust leaks can distort measurements and trigger offset errors

4. 4

   Ensure the correct voltage sensor is currently fitted (a 12-volt NOx sensor fitted to a 24-volt machine or vice versa will trigger errors)

5. 5

   Verify that the SCR system is dosing AdBlue/DEF correctly to rule out systemic SCR issues

1. 1

   Safety First

   Stop the engine and allow the exhaust system to cool completely, as surfaces can exceed 600°C (1,112°F). Wear gloves and safety glasses.

2. 2

   Disconnect Power

   Disconnect the machine's battery to avoid short circuits in the sensor's heater circuit.

3. 3

   Locate and Disconnect

   Locate the downstream NOx sensor (after the SCR catalyst) and disconnect its electrical connector.

4. 4

   Remove the Sensor

   Use a high-temperature rated NOx sensor socket or an O2 offset socket to carefully remove the faulty sensor. Use penetrating oil if the threads are seized.

5. 5

   Prepare the New Sensor

   Apply a high-temperature assembly paste to the threads of the new sensor only if approved by the OEM instructions.

6. 6

   Install and Torque

   Thread the new sensor into the exhaust pipe by hand to prevent cross-threading, then tighten using a torque wrench to OEM specifications (typically 35–55 Nm, depending on the exact engine model).

7. 7

   Reconnect

   Route the wiring harness safely away from extreme heat sources and reconnect the electrical plug. Reconnect the battery.

8. 8

   ECU Adaptation

   Connect an OBD-compatible diagnostic tester. Clear the stored fault codes and perform the required ECU adaptation/calibration for the new NOx sensor.

9. 9

   Validation

   Run a test/drive cycle to bring the exhaust up to operating temperature and verify that the sensor is communicating correctly and the code does not return.