Case IH 19009 (DTC 18DE) - ECU – Engine Control Unit (ECU): NH3 sensor short in heater circuit causing overheating

The NH3 (Ammonia) sensor relies on an internal electrical heater circuit to reach operating temperature. This fault code triggers when there is an electrical short in this heating circuit, causing the sensor to overheat.

• MAXXUM T4E
• 240 Series Axial-Flow Combines (7240, 8240, 9240)
• Farmall Series (110A, 120A, 130A, 140A, 110U Tier 4B)
• Steiger
• Magnum
• Farmlift (935)

1. 1

   Inspect the wiring harness connecting the NH3 sensor's dedicated module to the main engine harness for chafing, melted insulation, or shorting to ground.

2. 2

   Disconnect the harness at the NH3 module and inspect the pins for corrosion, moisture intrusion, or bent terminals.

3. 3

   Check the short wiring lead between the NH3 sensor probe and its mounting module for damage.

4. 4

   Verify if the fault remains active; if external wiring and connectors are intact, the short is internal to the sensor's heater circuit and the sensor must be replaced.

1. 1

   Cool Down

   Ensure the machine is turned off and the exhaust system (specifically the SCR muffler and catalyst) has cooled completely to prevent severe burns.

2. 2

   Locate the Sensor

   Locate the NH3 sensor assembly. The probe is threaded into the exhaust stream at the outlet pipe of the SCR muffler/catalyst. The sensor's control module is mounted on a bracket directly above or adjacent to the probe.

3. 3

   Disconnect Electronics

   Unplug the main vehicle wiring harness from the NH3 sensor module.

4. 4

   Remove the Module

   Remove the two bolts securing the NH3 sensor module to its mounting bracket.

5. 5

   Remove the Probe

   Using a wrench (typically 7/8" or 22mm), carefully unscrew the NH3 sensor probe from the SCR exhaust outlet pipe.

6. 6

   Install New Sensor

   Thread the new NH3 sensor probe into the exhaust pipe and tighten it to OEM specifications.

7. 7

   Mount New Module

   Mount the new NH3 sensor module to the bracket. (Note: OEM replacement kits usually include a new updated control module that must be installed alongside the new probe).

8. 8

   Reconnect

   Plug the main wiring harness into the new module.

9. 9

   Clear Codes

   Turn on the machine and use a diagnostic service tool to clear the inactive fault codes. (Most updated OEM kits are "Plug and Play" and do not require a software update if the provided module is used).