Case IH F-171-02 - ECU – Engine Control Unit (ECU): Plausibility check error of the third fresh air temperature sensor at cold start

During a cold start after an extended cold soak, the Engine Control Unit (ECU) compares temperature sensor readings. If the fresh air or oil temperature sensor reports a significantly different temperature from the others, it is flagged as implausible. This typically indicates internal resistance drift within the sensor or high resistance in the wiring harness.

• MAGNUM AFS Connect
• Farmall 40
• Farmall 45
• Farmall 50
• MX100
• MX110
• MX120
• MX135
• MXM120
• MXM130
• 9900 Series Harvesters

1. 1

   Connect a diagnostic service tool after the machine has been sitting overnight (cold soak). Review live data for Oil, Coolant, Fuel, and Fresh Air temperatures to identify which sensor is deviating from ambient temperature.

2. 2

   Verify the battery is fully charged and holding proper voltage during cranking, as low voltage can cause plausibility errors.

3. 3

   Inspect the wiring harness leading to the suspect sensor for corrosion at the connector pins, moisture intrusion, or chafed wires that could cause high resistance.

1. 1

   Disconnect the negative battery cable to prevent short circuits.

   Disconnect the negative battery cable to prevent short circuits.

2. 2

   Locate the faulty sensor.

   The transmission/engine oil temperature sensor is typically located on the side of the transmission housing or engine block; the fresh air temperature sensor is located on the intake piping or EGR circuit.

3. 3

   Unplug the electrical connector from the sensor.

   Clean the connector with contact cleaner and inspect for pushed-out pins.

4. 4

   Using a multimeter, measure the resistance across the sensor's pins.

   Compare the reading to the OEM resistance chart for your current ambient temperature.

5. 5

   If the sensor resistance is out of specification (drifted), use a wrench to remove the sensor.

   Have a catch pan ready if removing an oil temperature sensor, as a small amount of fluid may leak.

6. 6

   Install the new OEM replacement sensor and tighten to the manufacturer's torque specification.

   Install the new OEM replacement sensor and tighten to the manufacturer's torque specification.

7. 7

   Reconnect the electrical harness, ensuring it clicks securely into place.

   Reconnect the electrical harness, ensuring it clicks securely into place.

8. 8

   Reconnect the battery.

   Reconnect the battery.

9. 9

   Connect your diagnostic tool, clear the inactive fault codes, and perform a cold start to verify the code does not return.

   Connect your diagnostic tool, clear the inactive fault codes, and perform a cold start to verify the code does not return.