Case IH 19025 – Engine Control Module (ECM) / Dosing Control Unit (DCU): NH3 slip detected

This code indicates that unreacted ammonia (NH3) is passing through the Selective Catalytic Reduction (SCR) catalyst and exiting the tailpipe (a condition known as "ammonia slip"). In Tier 4 Final / Stage IV emissions systems, if too much DEF is injected, if the exhaust temperature is too low, or if the SCR catalyst is degraded, the excess ammonia cannot be absorbed and slips out.

• Tier 4b
• Puma (140-240)
• Optum (250-300)
• Maxxum
• Vestrum
• Farmall
• Luxxum
• 5140
• 8230
• 580N
• 590SN
• SR270
• SV300
• TR340

1. 1

   Check DEF/AdBlue Quality: Verify the DEF concentration is exactly 32.5% using a refractometer. Contaminated or incorrect DEF concentration causes the system to over-dose, leading to NH3 slip.

2. 2

   Perform a Dosing Valve Check (UDST): Run a Urea Dosing System Test via the diagnostic tool to check if the DEF injector is leaking or stuck open (permanent injection).

3. 3

   Inspect the SCR Catalyst: Check for heavy DEF crystallization/buildup at the injector nozzle and the entrance of the SCR catalyst.

4. 4

   Check NOx Sensor Accuracy: Ensure the downstream NOx sensor is not suffering from signal drift. A failing NOx sensor can falsely report high NH3 levels.

5. 5

   Check for Related Codes: Look for concurrent codes related to exhaust temperature sensors or NOx sensor plausibility (e.g., 19036, 19042), which can prevent the SCR from reaching the proper operating temperature.

1. 1

   Address DEF Fluid

   If the DEF is contaminated or degraded, drain the DEF tank, flush the system, and refill with fresh, ISO-compliant DEF.

2. 2

   Clean or Replace the DEF Dosing Valve

   Remove the DEF injector from the exhaust pipe. Inspect the tip for severe crystallization. Clean the mounting port. If the injector failed the leak test during diagnostics or is mechanically stuck open, replace the DEF Dosing Valve.

3. 3

   Perform a Forced Regeneration (Desorb)

   If the SCR catalyst is saturated with ammonia or crystallized DEF, perform a forced manual regeneration (often called a desoot or thermal management cycle) using the diagnostic tool. This raises the exhaust temperature to burn off deposits and release stored NH3.

4. 4

   Replace the Downstream NOx Sensor

   If the DEF injector is functioning perfectly and the SCR catalyst is clear, the downstream NOx sensor has likely failed or drifted. Replace the sensor.

5. 5

   Clear Codes and Test

   Clear the fault codes from the ECM/DCU. Run the machine under a heavy working load to ensure the SCR system reaches operating temperature and verify the code does not return.