Case IH 19356 (DTC 3FDE) - ECU – Engine Control Module (ECM) or Dosing Control Unit (DCU): Downstream NOx sensor signal and NH3 sensor signal plausibility check failure.

The control module has detected that the readings from the downstream Nitrogen Oxide (NOx) sensor and the Ammonia (NH3) sensor do not logically correlate with one another. This plausibility failure indicates that the Selective Catalytic Reduction (SCR) system cannot accurately verify the exhaust emissions, which is usually caused by a failing sensor, a wiring issue, or an actual chemical imbalance in the exhaust stream (such as poor DEF/AdBlue quality).

• MAXXUM T4E
• 240 Series Axial-Flow Combines (7240, 8240, 9240)
• Farmall Series (110U, 120U, 130A, 140A)
• Magnum
• Maxxum
• Steiger
• T7 Series
• WD2505 High-Speed Windrowers

1. 1

   Verify the quality and concentration of the Diesel Exhaust Fluid (DEF/AdBlue).

2. 2

   Inspect the wiring harnesses and connectors for both the Downstream NOx sensor and the NH3 sensor for melted wires, corrosion, or loose pins.

3. 3

   Check for exhaust leaks near the SCR muffler and catalyst.

4. 4

   Use a diagnostic tool to monitor live data (Note: NH3 sensor has a dew point warm-up cycle and will read 'not ready' during cold start).

5. 5

   Check for accompanying specific circuit codes (e.g., heater circuit open/short) to isolate the faulty sensor.

1. 1

   Safety First

   Ensure the machine is turned off, the battery is disconnected, and the exhaust system has completely cooled down to prevent severe burns.

2. 2

   Access the Sensors

   Locate the SCR muffler and catalyst assembly. Remove the SCR muffler upper heat shield and any exhaust clamps obstructing access to the sensors.

3. 3

   Disconnect Wiring

   Locate the Downstream NOx sensor and the NH3 sensor on the exhaust outlet pipe. Carefully disconnect their electrical harness connectors.

4. 4

   Remove the Sensor(s)

   Using the appropriate wrench, loosen the hexagonal nut at the base of the faulty sensor and remove it from the SCR muffler.

5. 5

   Install the New Sensor(s)

   Apply a high-temperature anti-seize compound to the threads of the new sensor (if not pre-applied by the manufacturer). Thread the sensor in by hand to prevent cross-threading, then tighten to the OEM specified torque.

6. 6

   Reassembly

   Reconnect the electrical harnesses. Reinstall the exhaust clamps and the SCR muffler heat shield.

7. 7

   System Reset

   Reconnect the battery. Turn the key switch ON to initiate the sensor warm-up (dew point) cycle. Clear the 19356 (3FDE) fault code using a diagnostic tool and run the engine to verify the repair.