Technical Description of OBD-II Trouble Code
Performance of Ambient Air Temperature Sensor
What does that signify?
This particular diagnostic trouble code is applicable to all engines equipped with OBDII, but it is more commonly observed in certain Audi, BMW, Chrysler, Dodge, Ford, Jeep, Mazda, Mitsubishi, and VW vehicles.
The Ambient Air Temperature (AAT) sensor converts the ambient air temperature into an electrical signal for the Powertrain Control Module (PCM). This input is used to adjust the operation of the air conditioning system and display the outside air temperature.
The PCM receives this input along with possibly two more inputs: Intake Air Temperature (IAT) and Engine Coolant Temperature Sensor (ECT). When the ignition switch is first turned on after a long cool down period, the PCM compares the AAT sensor voltage to the IAT/ECT sensor readings. If these inputs vary too much, this code is set. It also examines the voltage signals from these sensors to determine their accuracy when the engine is fully warmed up. Although this code is usually triggered by electrical issues, mechanical faults should not be disregarded. These mechanical issues may include improper mounting location of the sensor or not mounting the sensor properly (leaving it hanging by the wiring harness), among others.
Troubleshooting steps may differ depending on the manufacturer, type of AAT sensor, and wire colors.
Symptoms
The symptoms may include:
Causes
Possible causes of a P0071 trouble code may include:
Possible Solutions
A good starting point is always to check for technical service bulletins (TSB) for your particular vehicle. Your issue may be a known problem with a known solution provided by the manufacturer, which can save you time and money during diagnosis.
Next, locate the AAT sensor on your specific vehicle. This sensor is usually situated in front of the radiator behind the grille or front bumper area. Once found, visually inspect the connectors and wiring. Look for any scraping, rubbing, bare wires, burn spots, or melted plastic. Separate the connectors and carefully examine the terminals (the metal parts) inside the connectors. Check if they appear burned or have a green tint indicating corrosion. If necessary, use electrical contact cleaner and a plastic bristle brush to clean the terminals. Allow them to dry and apply electrical grease where the terminals make contact.
The most common failure is related to connections, with a failed sensor being the second most common due to its harsh environment.
While checking the connections, you can test the sensor using a Digital Volt-Ohm Meter (DVOM). With the ignition switch off, disconnect the sensor and connect the Red (Positive) lead of the DVOM to one post on the sensor and the Black (Negative) lead of the DVOM to the other post. Determine the temperature of the sensor (the outside temperature) and find the corresponding resistance on the chart. This is the resistance in ohms that your DVOM should display. A reading of either 0 ohms or infinite resistance (usually indicated by the letters OL) indicates a failed sensor.
If you have a scan tool, clear the diagnostic trouble codes from memory and check if this code reappears. If it does not, then the issue was most likely related to the connections.
If the P0071 code does reappear, further testing of the AAT sensor and its associated circuits is necessary. Typically, there are 2 wires connected to the AAT sensor. With the ignition switch off, disconnect the harness going to the AAT sensor. Turn the ignition switch on. Using a scan tool to access PCM data (assuming the module receiving the AAT sensor input is the PCM or another module towards the front of the vehicle that can transmit the AAT sensor data over a bus network), read the AAT sensor temperature or voltage. It should read 5 volts or a value other than the ambient air temperature (a very low temperature) in degrees. Next, turn the ignition switch off, connect a jumper wire across the two terminals inside the harness connector going to the AAT sensor, then turn the ignition switch on. It should read close to 0 volts or a value other than the ambient air temperature (a very high temperature) in degrees. If there is no 5 volts to the sensor or if you observe no change, repair the wiring from the PCM to the sensor or consider the possibility of a faulty PCM.
If all previous tests have been successful and you continue to encounter a P0071 code, it most likely indicates a failed AAT sensor. However, a failed control module cannot be ruled out until the AAT sensor has been replaced. If you are unsure, seek assistance from a trained automotive diagnostician. PCMs must be programmed or calibrated to the vehicle in order to be installed correctly.
