What is the P0071 code?
The P0071 error code is a generic powertrain-related code that indicates a performance issue with the Ambient Air Temperature (AAT) sensor. This sensor, found on most OBDII-equipped vehicles (including Audi, BMW, Chrysler, Dodge, Ford, Jeep, Mazda, Mitsubishi, and VW), is responsible for converting the outside air temperature into an electrical signal sent to the Powertrain Control Module (PCM).
This data is crucial for regulating the air conditioning system and displaying the outside temperature on the dashboard. The PCM compares the AAT sensor signal with those from the Intake Air Temperature (IAT) sensor and the Engine Coolant Temperature (ECT) sensor upon startup, after a cooldown period. An abnormal discrepancy between these values triggers the P0071 code.
Symptoms of the P0071 code
Several signs can alert you to a malfunction of the AAT sensor:
- The malfunction indicator lamp (check engine) is illuminated.
- Poor operation or inefficiency of the air conditioning system.
- Incorrect or missing display of the outside temperature on the dashboard or console.
Possible causes of the P0071 error
The origins of this code are primarily electrical, but mechanical faults are also possible:
- Open or shorted signal circuit (to ground or to voltage).
- Faulty AAT sensor (the most common cause after connection issues).
- Incorrect mounting location or sensor left hanging (mechanical defect).
- PCM failure (rare).
How to diagnose and fix a P0071 code?
Always start by consulting your manufacturer’s Technical Service Bulletins (TSBs) to check for any known issues.
1. Visual Inspection
Locate the AAT sensor, usually positioned in front of the radiator. Inspect the connector and wiring for any signs of wear, corrosion (greenish tint), or burning. Clean the terminals with contact cleaner and a non-abrasive brush if necessary, then apply dielectric grease.
2. Sensor Testing
Using a multimeter (DVOM) in ohmmeter mode, measure the resistance of the (disconnected) sensor and compare it to the reference values for the ambient temperature. A reading of 0 Ω or infinite resistance (OL) indicates a faulty sensor.
3. Circuit Testing
Reconnect the sensor and use a diagnostic tool to read the voltage or temperature reported by the sensor to the PCM. With the ignition on, the voltage should be around 5V. By shorting the two terminals of the harness connector (ignition off), the reading should drop close to 0V. If these values are not observed, the electrical circuit or the PCM is at fault.
4. Replacement and Conclusion
If the previous tests point to the sensor, replace it. Then, clear the codes with your diagnostic tool and perform a road test to verify that the code does not return. If the problem persists, the PCM might be faulty, but this is rare. If in doubt, consult an automotive professional.
Need help diagnosing your vehicle? Consult a partner garage or invest in a reliable OBD2 diagnostic tool to perform these checks yourself.