Case IH 3549 (DTC 1C54) - ECU – Engine Control Unit (ECU): Intake air humidity sensor voltage is higher than expected for too long

The intake air humidity sensor monitors the humidity and temperature of the incoming air. The engine uses this data to correct the estimation of raw engine NOx production and properly adjust the Diesel Exhaust Fluid (DEF)/AdBlue® injection rate. This fault code triggers when the sensor's signal voltage exceeds the maximum expected limit for an extended period, which typically indicates a short circuit to an external power source, a defective sensor, or a sensor tip saturated with water droplets.

• MAXXUM T4E
• 240 Series Axial-Flow Combines (7240, 8240, 9240)
• Magnum 250-380 series
• Puma series
• Steiger series
• WD1505

1. 1

   Check the sensor tip for physical water droplets or excessive moisture buildup, which is a known cause for this specific high-voltage fault.

2. 2

   Inspect the wiring harness between the humidity sensor and the ECU. Look for chafed wires causing a short circuit to an external voltage source (SRC High).

3. 3

   Disconnect the sensor and check for water intrusion, corrosion, or bent pins inside the connector.

4. 4

   Verify there are no disturbed CAN signal transmission messages accompanying this code, which could indicate a broader network issue.

1. 1

   Locate the Sensor

   Find the intake air humidity sensor. It is located in the inlet air plumbing between the air filter and the turbocharger.

2. 2

   Inspect and Dry

   Disconnect the sensor harness. Remove the sensor from the intake plumbing and inspect the probe. If it is covered in water droplets, dry it thoroughly, reinstall, and retest.

3. 3

   Test the Circuit

   Turn the key to the ON position (engine off). Use a multimeter to probe the sensor connector and verify that the reference voltage and ground are within specification, ensuring there is no short to a 12V/high-voltage source on the signal wire.

4. 4

   Replace the Sensor

   If the wiring is intact, the connector is clean, and the sensor is dry but the code remains active, the sensor itself has failed internally and must be replaced.

5. 5

   Clear Codes

   Install the new sensor, reconnect the harness, and use the machine's diagnostic display or a service tool to clear the inactive fault codes. Run the engine to ensure the code does not return.