Case IH F-3226-31 - ECU – Engine Control Unit (ECU): Downstream NOx sensor supply voltage is out of range

The engine controller is detecting an electrical fault, voltage irregularity, or internal failure preventing the downstream NOx sensor (located after the SCR catalyst) from operating or communicating correctly.

• MAGNUM AFS Connect
• Farmall (110A-140A)
• Maxxum (115-150)
• Puma (150-220)
• Magnum (180-380)
• Steiger 4WD/Quadtrac series
• 240 Series Axial-Flow Combines (7240, 8240, 9240)
• Austoft 9900 sugarcane harvesters

1. 1

   Check vehicle supply voltage to ensure the battery and alternator are functioning correctly and providing stable voltage (voltage spikes or low voltage during cold starts can trigger this code)

2. 2

   Inspect the wiring harness leading to the downstream NOx sensor for chafing, melted wires, or poor connections that could cause a short circuit to ground or battery

3. 3

   Disconnect the downstream NOx sensor and inspect the connector pins for corrosion, moisture, or damage

4. 4

   Test for internal failure within the NOx sensor itself (such as an internal short or heater circuit failure) if the wiring harness and supply voltage test within normal limits

1. 1

   Turn off the machine and disconnect the battery

   Ensure safety while working on the electrical and exhaust systems.

2. 2

   Locate the downstream NOx sensor

   The sensor is mounted on the exhaust pipe after the SCR catalyst.

3. 3

   Trace the sensor's pigtail to the chassis wiring harness connector

   Disconnect the plug.

4. 4

   Perform the diagnostic wiring checks

   Check vehicle supply voltage, inspect the wiring harness, and check connectors as outlined in the diagnostic checks.

5. 5

   Remove the downstream NOx sensor

   If the wiring is intact and the code persists (or if the sensor is visibly damaged/internally shorted), remove the downstream NOx sensor from the exhaust pipe using a specialized NOx/O2 sensor socket.

6. 6

   Apply a high-temperature anti-seize compound

   Apply to the threads of the new sensor (if not pre-applied by the manufacturer).

7. 7

   Install the new downstream NOx sensor

   Torque to OEM specifications.

8. 8

   Reconnect the sensor to the wiring harness

   Ensure the locking tab clicks into place.

9. 9

   Clear the fault code

   Reconnect the battery, turn the key to the ON position, and use a diagnostic tool (such as the Case IH Electronic Service Tool) to clear the F-3226-31 fault code.

10. 10

    Verify the repair

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