Case IH 19009 – Engine Control Unit (ECU) / Electronic Diesel Control (EDC): NH3 sensor short in heater circuit causing overheating

The Ammonia (NH3) sensor is a critical component of the Selective Catalytic Reduction (SCR) aftertreatment system, used to detect the concentration of ammonia in the exhaust after the SCR catalyst to ensure the system is reducing emissions properly. The sensor contains an internal heater to reach operating temperature; a short circuit in this heater causes the sensor to overheat, rendering its data invalid and triggering the fault.

• Tier 4b
• 240 Series Axial-Flow Combines
• 7240
• 8240
• 9240
• Steiger/Quadtrac series
• 620
• Farmall series
• 110A
• 120A
• 130A
• 140A
• Puma
• Maxxum series
• Patriot series
• 3340

1. 1

   Inspect the wiring harness and connectors leading to the NH3 sensor for melting, chafing, or broken wires that could cause a short circuit.

2. 2

   Verify the voltage supply to the NH3 sensor module and ensure the CAN controller is properly connected to the network without undervoltage.

3. 3

   Disconnect the slide-lock connector and use a digital multimeter to measure the trim resistance of the NH3 sensor probe. The trim resistance must be between 500 and 2000 ohms. If it falls outside this range, the probe is defective.

1. 1

   Disconnect Power

   Turn off the machine and disconnect the batteries (remove the negative cable first, then the positive) to prevent arcing and electrical damage.

2. 2

   Locate the Sensor

   Locate the NH3 sensor on the SCR muffler/catalyst. You may need to remove the SCR muffler upper heat shield to access it.

3. 3

   Disconnect Wiring

   Disconnect the slide-lock electrical connector from the NH3 sensor module.

4. 4

   Remove the Sensor

   Carefully unscrew and remove the Ammonia (NH3) sensor probe from the exhaust pipe/SCR catalyst.

5. 5

   Install the New Sensor

   Apply high-temperature anti-seize compound to the threads of the new sensor (if not already pre-applied by the manufacturer) and thread it into the SCR catalyst. Tighten to OEM specifications.

6. 6

   Reassembly

   Reconnect the electrical harness to the sensor module. Reinstall any heat shields that were removed.

7. 7

   Restore Power

   Reconnect the batteries (positive first, negative last).

8. 8

   Clear Codes

   Use a diagnostic tool to clear the 19009 / 18DE fault code and run the engine to verify that the SCR system is functioning normally without derating.