Case IH P0071 – PCM / ECM / Body Control Module / Instrument Cluster: Ambient Air Temperature Sensor Range/Performance

The Powertrain Control Module (PCM) or Engine Control Module (ECM) has detected that the signal from the Ambient Air Temperature (AAT) sensor is outside the expected range or is inconsistent with readings from the Intake Air Temperature (IAT) and Engine Coolant Temperature (ECT) sensors. The computer compares these sensor readings, especially after the machine has been off for a period, to ensure they are all within a similar range. The AAT sensor is a thermistor that monitors surrounding air temperature; its failure or inaccurate data can negatively affect the automatic climate control system, fuel trim calculations, idle control, and overall engine performance.

• CX Series
• MX Series
• Magnum Series
• STX Series

1. 1

   Visually inspect the AAT sensor for physical damage, corrosion, or contamination from dirt and debris.

2. 2

   Check the sensor's electrical connector for a secure fit and inspect for any signs of corrosion or bent pins.

3. 3

   Examine the wiring harness leading to the AAT sensor for cuts, abrasions, or heat damage. Perform a wiggle test on the harness while monitoring the sensor's reading on a scan tool to identify intermittent connections.

4. 4

   Use a diagnostic scan tool to observe live data from the AAT sensor after a cold soak (machine off for several hours). Compare it to IAT and ECT readings; they should be very close. Extreme stuck values (like -40°F or 250°F) indicate a sensor or circuit problem.

5. 5

   Disconnect the AAT sensor and use a multimeter to measure resistance across its terminals. Gently warm the sensor with a hairdryer to verify the resistance changes, and compare values to service manual specifications.

6. 6

   With the ignition on and sensor disconnected, check for a 5-volt reference voltage at the sensor's connector and perform a continuity test on the wiring between the connector and the control module.

1. 1

   Confirm fault and locate sensor

   Use a diagnostic scanner to confirm P0071 is the active fault code. Locate the AAT sensor, which is commonly found behind the front grille, near the radiator, in a side mirror, or along the frame rail on heavy-duty equipment.

2. 2

   Repair wiring or connections

   If diagnostic checks reveal damaged wiring, open circuits, or shorts, repair or replace the affected section of the harness. Ensure all electrical connections are clean and secure.

3. 3

   Replace AAT sensor

   If the wiring is intact but the sensor fails the resistance test, shows an open circuit, or outputs extreme out-of-range values on the scan tool, replace the Ambient Air Temperature sensor.

4. 4

   Test or replace control module

   In the rare event that both the sensor and wiring test perfectly, the issue may reside within the control module. This may require reprogramming or replacement by a qualified technician.

5. 5

   Clear code and verify repair

   Use a scan tool to clear the P0071 fault code. Start the machine, monitor the AAT sensor data to ensure accurate readings, and perform a test drive to confirm the fault does not return.