Case IH F-3234-05 - ECU – Engine Control Unit (ECU): Downstream NOx sensor internal failure (Open Circuit Error)

The system has detected an open electrical circuit within the downstream Nitrogen Oxide (NOx) sensor or its measurement circuit. The downstream NOx sensor is located directly after the Selective Catalytic Reduction (SCR) catalyst and measures the pollutants exiting the tailpipe to ensure the emissions system is dosing Diesel Exhaust Fluid (DEF) correctly.

• MAGNUM AFS Connect
• Magnum series (e.g., Magnum AFS Connect, 180-370)
• Steiger series (e.g., 350-600, Quadtrac)
• Maxxum series
• Puma series
• Axial-Flow Combines (e.g., 240 series: 7240, 8240, 9240)
• Farmlift Telehandlers
• Case Wheel Loaders (e.g., 1021F, 1121F)

1. 1

   Inspect the wiring harness and connectors between the downstream NOx sensor and the ECU for any physical damage, chafing, melting, or loose connections.

2. 2

   Disconnect the sensor and inspect the pins for corrosion, moisture, or bent terminals.

3. 3

   Verify that the sensor is receiving the correct power supply voltage and has a solid ground connection (an open circuit in the main harness triggers the same code).

4. 4

   Check for accompanying CAN timeout errors (e.g., 3533) or heater circuit errors (e.g., 3232-05) which indicate the issue is likely in the main wiring harness or a shared relay rather than the sensor itself.

1. 1

   Safety First

   Turn off the engine and allow the exhaust system to cool completely. The SCR catalyst and exhaust pipes operate at extremely high temperatures.

2. 2

   Power Down

   Turn off the machine's battery disconnect switch to prevent electrical shorts.

3. 3

   Locate the Sensor

   Locate the downstream NOx sensor, which is threaded into the exhaust pipe directly after the SCR catalyst.

4. 4

   Disconnect Wiring

   Unplug the electrical connector for the NOx sensor from the machine's main wiring harness.

5. 5

   Remove the Sensor

   Using a specialized NOx/O2 sensor socket or a suitable wrench, carefully unscrew the sensor from the exhaust pipe. (Note: Penetrating oil may be required if the sensor threads are seized from heat cycling).

6. 6

   Prep the Port

   Clean the mounting threads in the exhaust pipe to ensure the new sensor seats correctly.

7. 7

   Install the New Sensor

   Thread the new downstream NOx sensor into the exhaust pipe and tighten it to the OEM specified torque.

8. 8

   Route the Wiring

   Reconnect the electrical connector. Ensure the sensor wiring is routed securely away from extreme heat sources and moving parts.

9. 9

   Clear Codes

   Reconnect the battery power. Connect a diagnostic tool (such as the Case IH EST) to the machine and clear the inactive fault codes. A manual code clear is usually required after fitting a new NOx sensor to reset the SCR system.

10. 10

    Test Run

    Start the engine and allow it to reach operating temperature to verify the code does not return and the SCR system is functioning properly.