Case IH 19777 – Engine Control Unit (ECU) or NH3 Control Unit: NH3 and NO2 signal from Ammonia sensor are not valid

The control module is receiving an erratic, out-of-range, or otherwise invalid signal from the Ammonia (NH3) sensor, which monitors residual ammonia (ammonia slip) downstream of the Clean Up Catalyst.

• Tier 4b
• Axial-Flow 6150
• Axial-Flow 7150
• Axial-Flow 7250
• Axial-Flow 8250
• Magnum 220
• Puma 200
• Farmall U
• Farmall 120 U
• Farmall 130 A
• N-series TLBs
• 821F Wheel Loader
• 921F Wheel Loader

1. 1

   Inspect the 8-pin electrical connector at the NH3 sensor and the wiring harness leading back to the ECU for open circuits, chafed wires, or corrosion.

2. 2

   Check for exhaust leaks near the SCR muffler and catalyst, which can introduce unmetered oxygen and skew sensor readings.

3. 3

   Ensure the Diesel Exhaust Fluid (DEF/AdBlue) is uncontaminated, as poor quality or a malfunctioning doser can cause excessive ammonia slip.

4. 4

   Note that there is no field-test method to verify the sensing cell portions of the NH3 sensor; if wiring is intact, the sensor or control unit must be replaced.

1. 1

   Turn off the machine and disconnect the battery

   This prevents electrical shorts during the repair.

2. 2

   Allow the exhaust system to cool completely

   Tier 4B SCR systems maintain extremely high temperatures (often above 270°C).

3. 3

   Locate the NH3 sensor

   The sensor is mounted downstream of the SCR muffler / Clean Up Catalyst.

4. 4

   Remove heat shields if necessary

   If necessary for access, remove the SCR muffler and catalyst upper heat shields.

5. 5

   Disconnect the electrical connector

   Disconnect the 8-pin electrical connector from the NH3 sensor.

6. 6

   Remove the NH3 sensor

   Unscrew and remove the Ammonia (NH3) sensor from the exhaust pipe.

7. 7

   Install the new NH3 sensor

   Apply high-temperature anti-seize to the threads of the new sensor (if not pre-applied) and install it into the exhaust pipe, tightening to the OEM torque specification.

8. 8

   Reconnect connector and heat shields

   Reconnect the 8-pin electrical connector and reinstall any heat shields that were removed.

9. 9

   Clear fault code and verify repair

   Reconnect the battery, turn on the machine, and use the Case IH Electronic Service Tool (EST) to clear the fault code and verify the repair.