Case IH 3937 – Engine Control Unit (ECU): Air Temp in AFS: Signal range high on the duty cycle signal

This fault indicates that the Engine Control Unit (ECU) is receiving a Pulse Width Modulated (PWM) duty cycle signal from the Air Flow Sensor's (AFS) integrated intake air temperature sensor that exceeds the maximum expected range.

• Case IH Magnum
• Case IH Steiger
• Case IH Quadtrac
• Case IH Optum
• Case IH Puma
• Case IH Axial-Flow combines

1. 1

   Inspect the wiring harness leading to the Air Flow/Intake Air Temperature sensor for chafed wires, melted insulation, or pinch points.

2. 2

   Disconnect the sensor and inspect the pins for corrosion, moisture, or pushed-back terminals.

3. 3

   Check the signal wire between the sensor connector and the ECU for an open circuit or a short to voltage.

4. 4

   Verify that the sensor is receiving the correct reference voltage and has a solid ground connection to the ECU.

5. 5

   Check if other 5V reference codes or temperature codes (e.g., 3938, 3939, 3941) are active, which would indicate a shared ground failure or a damaged ECU sensor supply circuit.

1. 1

   Power Down

   Turn off the machine, remove the key, and disconnect the negative battery terminal to prevent electrical spikes.

2. 2

   Locate the Sensor

   Locate the Air Flow Sensor / Intake Air Temperature sensor, which is typically mounted on the engine air intake manifold or the intake piping just after the air filter.

3. 3

   Disconnect and Remove

   Unplug the electrical connector. Depending on the specific engine model, remove the retaining bolts or unthread the sensor from the intake manifold.

4. 4

   Install the New Sensor

   Insert the replacement sensor. If it is a threaded sensor, apply a small amount of sensor-safe thread sealant (if not pre-applied) and tighten to the OEM torque specification. If it is a flange-style sensor, ensure the O-ring is seated correctly and tighten the retaining screws.

5. 5

   Reconnect

   Plug the electrical connector back in, ensuring the locking tab clicks into place.

6. 6

   Clear Codes

   Reconnect the battery. Turn the ignition on and use the Case IH AFS Pro display or the EASy diagnostic tool to clear the inactive fault codes.

7. 7

   Test Run

   Start the engine and allow it to reach operating temperature. Verify that the check engine light remains off and code 3937 does not return.