Case IH 3559 – Engine Control Unit (ECU): Intake Air Humidity sensor failure

The intake air humidity sensor is reporting an implausible voltage, an open circuit, or a short circuit. This prevents the ECU from accurately estimating raw engine NOx production, which dictates the precise amount of DEF/AdBlue needed to be injected into the exhaust stream.

• Magnum Tractors (235, 250, 310, 340, 380 series)
• Steiger Tractors
• Puma series tractors
• Maxxum series tractors
• Axial-Flow Combines (140 and 240 series, 7240, 8240, 9240)

1. 1

   Visual Inspection for Moisture: Check if the sensor element is heavily saturated with water droplets from extreme environmental humidity or water ingestion.

2. 2

   Wiring and Connector Check: Inspect the electrical connector at the sensor for corrosion, pushed-back pins, or moisture intrusion. Trace the wiring harness back toward the ECU to check for chafed or broken wires.

3. 3

   Voltage Test: Using a multimeter, verify that the sensor is receiving the proper 5V reference voltage and ground from the ECU with the key in the ON position.

4. 4

   Intake Inspection: Inspect the air intake plumbing between the air filter and turbocharger for unmetered air leaks, blockages, or excessive dust bypass that could have fouled the sensor element.

1. 1

   Turn off machine

   Turn off the machine, remove the key, and allow the engine to cool.

2. 2

   Locate sensor

   Open the engine cowling and locate the intake air humidity sensor. It is mounted directly into the plastic or metal air intake tube situated between the main engine air filter housing and the turbocharger inlet.

3. 3

   Disconnect harness

   Depress the locking tab and disconnect the electrical wiring harness from the sensor.

4. 4

   Remove fasteners

   Remove the mounting screws or retaining clip securing the sensor to the intake plumbing.

5. 5

   Remove sensor

   Carefully pull the sensor straight out of the intake tube.

6. 6

   Clean mounting port

   Wipe the mounting port clean with a lint-free rag, being extremely careful not to knock any dirt or debris into the open engine intake.

7. 7

   Lubricate O-ring

   Lubricate the O-ring on the new sensor with a tiny amount of clean engine oil to prevent tearing.

8. 8

   Install new sensor

   Insert the new sensor into the intake tube, ensuring it seats fully and the O-ring seals the port (a bad seal will allow unfiltered air and dust into the turbocharger).

9. 9

   Reinstall fasteners and harness

   Reinstall the mounting screws/clip and reconnect the electrical harness.

10. 10

    Clear codes and verify

    Connect a diagnostic tool (such as the CNH Electronic Service Tool or an aftermarket equivalent like Jaltest) to clear the inactive fault codes. Start the engine and verify the code does not return.