Case IH 19504 – Engine Control Unit (ECU) or Aftertreatment Control Module (ACM): Downstream NOx Sensor: NOx signals invalid

This fault code indicates that the Engine Control Unit (ECU) or Aftertreatment Control Module (ACM) is receiving an implausible, erratic, or out-of-range signal from the downstream Nitrogen Oxide (NOx) sensor.

• Tier 4b
• T7 Series
• N-Series Backhoe Loaders (580N, 580SN, 580SN WT, 590SN)
• G-Series Wheel Loaders (1021G)

1. 1

   Inspect the downstream NOx sensor's wiring harness and the 5-cavity female wire connector for melted wires, chafing, or corrosion inside the connector pins.

2. 2

   Inspect the exhaust piping between the engine and the downstream NOx sensor for leaks, which can introduce outside oxygen and skew readings.

3. 3

   Verify that the Diesel Exhaust Fluid (DEF) is uncontaminated and that the DEF doser/injector is not clogged.

4. 4

   Look for accompanying upstream NOx sensor codes (e.g., 19507, 19508) or SCR temperature codes (e.g., 19506) which indicate a main aftertreatment harness issue or blown fuse.

1. 1

   Preparation

   Turn off the machine, remove the key, and disconnect the battery to prevent electrical shorts. Allow the exhaust system to cool completely.

2. 2

   Locate the Sensor

   Locate the downstream NOx sensor, which is threaded into the exhaust pipe directly after the SCR catalyst canister.

3. 3

   Disconnect

   Unplug the sensor's electrical connector from the main engine/aftertreatment wiring harness.

4. 4

   Removal

   Use a specialized NOx sensor socket (typically 22mm) or a wrench to remove the sensor from the exhaust bung. If the sensor is seized, apply penetrating oil and allow it to soak before attempting to break it loose.

5. 5

   Clean the Threads

   Inspect the exhaust pipe bung threads. Use a thread chaser to clean out any soot or rust to ensure the new sensor seats correctly.

6. 6

   Installation

   Apply a small amount of high-temperature anti-seize compound to the threads of the new sensor (avoid getting any on the sensor tip). Thread the new sensor in by hand to prevent cross-threading, then tighten to the OEM torque specification.

7. 7

   Reconnect

   Route the sensor wiring away from extreme heat sources and moving parts, then plug the connector back into the harness.

8. 8

   Reset and Test

   Reconnect the battery. Connect a diagnostic scan tool to clear fault code 19504. You may need to perform a reset of the NOx sensor adaptations in the ECU. Start the engine and perform a stationary regeneration or test drive to ensure the code does not return.