The AAT (Ambient Air Temperature Sensor) changes the outside air temperature into an electrical signal that it sends to the PCM (Powertrain Control Module). At the same time, the PCM uses this signal to display the outside air temperature and modify the air conditioning operation.
The PCM also receives signals from the ECT (Engine Coolant Temperature) Sensor and IAT (Intake Air Temperature) Sensor, which are compared to the AAT Sensor once the ignition switch is turned on after a cool-down period. If there is a narrow range of variation in all of these inputs, the DTC P0071 OBDII code will be set and the Check Engine light will turn on.