Case IH 19421 – Engine Control Module (ECM) or Vehicle Control Module (VCM) / Dosing Control Unit (DCU): Downstream NOx sensor failed negative drift test monitoring

This code indicates that the downstream Nitrogen Oxide (NOx) sensor—located in the exhaust stream after the Selective Catalytic Reduction (SCR) catalyst—is reading lower than expected (negative drift) during a self-test or monitoring cycle. This means the sensor's internal calibration has drifted out of its acceptable range, or there is an anomaly in the exhaust gas composition reaching the sensor.

• Tier 4b
• Farmall
• Magnum
• Combines
• 580N
• 580SN
• 590SN

1. 1

   Inspect the wiring harness and the connector to the downstream NOx sensor for chafing, heat damage, corrosion, or loose pins

2. 2

   Check for any exhaust leaks between the engine block and the downstream NOx sensor (fresh air can dilute gases and cause a false negative drift reading)

3. 3

   Verify that the Diesel Exhaust Fluid (DEF) dosing system is operating correctly (over-dosing or under-dosing can skew NOx levels)

4. 4

   Use the CNH Electronic Service Tool (EST) to monitor downstream NOx sensor readings in real-time and compare them to the upstream sensor

1. 1

   Preparation

   Turn off the machine, remove the key, and disconnect the battery to prevent electrical shorts. Ensure the exhaust system has completely cooled down.

2. 2

   Locate the Sensor

   Locate the downstream NOx sensor on the exhaust pipe, positioned after the SCR catalyst.

3. 3

   Disconnect

   Unplug the electrical connector for the NOx sensor from the machine's wiring harness.

4. 4

   Removal

   Use a specialized NOx sensor socket or a line wrench to carefully remove the sensor from the exhaust pipe. Note: If the sensor is seized, apply a high-quality penetrating oil and allow it to soak to prevent stripping the exhaust bung threads.

5. 5

   Clean the Port

   Clean the mounting boss threads in the exhaust pipe using a thread chaser if necessary.

6. 6

   Installation

   Apply a high-temperature anti-seize compound to the threads of the new NOx sensor (many OEM sensors come with this pre-applied). Thread the sensor in by hand to prevent cross-threading, then torque it to the manufacturer's specification.

7. 7

   Reconnect

   Plug the new sensor's electrical connector back into the wiring harness.

8. 8

   Relearn/Reset

   Reconnect the battery. Connect the CNH EST diagnostic tool, clear the 19421 fault code, and perform any required NOx sensor reset or adaptation procedures so the ECM recognizes the new sensor's baseline calibration.