P0071 - Ambient Air Temperature Sensor Range/Performance
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System
Powertrain (Engine & Gearbox)
Related Fault Codes
Technical Description
P0071 is set when the Engine Control Module (ECM) detects a rationality issue with the Ambient Air Temperature (AAT) sensor signal. The ECM compares the AAT sensor reading with the Intake Air Temperature (IAT) and Engine Coolant Temperature (ECT) sensors after a cold soak (engine off for 6+ hours). If the AAT value deviates significantly from the other sensors (typically >10-20°C), the PCM flags a performance fault. This data is used for climate control, fuel strategy, and EGR/DPF management.
Possible Causes
Faulty Ambient Air Temperature (AAT) sensor
Corrosion or poor connection in the sensor connector
Stuck engine thermostat (interferes with sensor rationality checks)
Wiring harness damage (short or open circuit)
ECM software or hardware internal fault
Symptoms
Recommended Solution
- Visual Inspection: Check the AAT sensor (usually located behind the front grille or in side mirrors) for damage. Inspect the connector for corrosion.
- Resistance Test (Ohms): Disconnect the sensor and measure resistance. At 68°F (20°C), it should typically be 2k-5k Ohms (refer to factory specs). Infinite resistance indicates an open circuit.
- Voltage Test: With ignition ON, check for 5V reference voltage at the sensor connector harness side.
- Live Data Analysis: Use a scan tool to monitor AAT, IAT, and ECT after a cold soak. All three should be within 3°C of each other.
- Action: Replace sensor if resistance is out of spec or if values fluctuate erratically.
Mechanic's Tip
Before replacing expensive components, always double-check wiring and connections. 80% of all sensor faults are actually due to corroded contacts or broken wires.
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