Case IH F-3232-06 - ECU – Engine Control Unit (ECU) / Engine Control Module (ECM): Downstream NOx sensor internal failure (Heater Short Circuit Error)

The FMI 6 designation specifically means Current Above Normal or Grounded Circuit (a short circuit). The downstream (or outlet) NOx sensor relies on an internal heating element to reach the high operating temperatures required to accurately measure nitrogen oxide levels in the exhaust gas after it passes through the Selective Catalytic Reduction (SCR) system. The system has detected a short circuit in this heater's power stage, which prevents the sensor from functioning correctly.

• MAGNUM AFS Connect
• Axial-Flow Combines (e.g., 240 Series)
• Magnum series tractors
• Maxxum series tractors
• Farmall series tractors
• Titan Floaters (e.g., 3530, 4530)

1. 1

   Inspect the wiring harness between the ECU and the downstream NOx sensor for chafing, melting, or pinched wires that could cause a short to ground or a short to the battery.

2. 2

   Disconnect the downstream NOx sensor connector and inspect the pins for water intrusion, corrosion, or bent terminals.

3. 3

   Check the sensor supply voltage and ground circuits to ensure the short is not located on the chassis/harness side.

4. 4

   If the harness and connectors are in good condition, the short circuit is internal to the NOx sensor heater, which is the most common cause of this specific failure.

1. 1

   Turn off the machine, ensure the exhaust system has completely cooled down, and disconnect the battery for safety.

   Safety precaution before beginning work on the exhaust system.

2. 2

   Locate the downstream NOx sensor.

   It is mounted on the exhaust pipe/silencer outlet, downstream of the SCR catalyst.

3. 3

   Disconnect the electrical connector for the downstream NOx sensor from the main harness.

   Prepare the sensor for removal.

4. 4

   Using a specialized NOx/O2 sensor socket, carefully unscrew and remove the faulty NOx sensor from the exhaust pipe.

   Remove the defective sensor.

5. 5

   Inspect the mounting bung on the exhaust pipe and clean the threads if necessary.

   Ensure a clean surface for the new sensor.

6. 6

   Apply a high-temperature anti-seize compound to the threads of the new NOx sensor.

   Only apply if it does not come pre-applied from the factory. Do not get anti-seize on the sensor probe itself.

7. 7

   Thread the new downstream NOx sensor into the exhaust bung and tighten to the OEM torque specification.

   Install the new sensor securely.

8. 8

   Route the sensor wire safely away from extreme heat sources and reconnect the electrical connector.

   Ensure the connector clicks and locks into place.

9. 9

   Reconnect the battery.

   Restore power to the machine.

10. 10

    Connect a diagnostic tool to clear the inactive fault codes.

    Start the engine and allow it to reach operating temperature to verify the heater circuit operates correctly and the code does not return.