Theory Of Operation

The Ambient Air Temperature (AAT) Sensor is a variable resistor that functions as a normal two wire, 5-Volt Sensor. The Body Control Module (BCM) supplies the AAT Sensor with a 5-Volt reference and a filtered ground (return) circuit. When the ambient air temperature is low, the sensor resistance is high. When the ambient air temperature is high, the sensor resistance is low. For the AAT Sensor diagnostic, the BCM internally monitors the voltage on the signal circuit, which is converted into a temperature reading. The AAT Sensor signal reading is a bussed message from the BCM to the Powertrain Control Module (PCM). If the voltage reads above the high calibrated threshold, or below the low calibrated threshold, a circuit high or low fault is set. The AAT Sensor rationality diagnostic runs under cold start conditions, following a start to run delay time and compares the outputs of the AAT, Engine Coolant Temperature (ECT), and Intake Air Temperature (IAT) Sensors. If the ECT and IAT Sensors agree and the AAT Sensor does not agree, the AAT is declared as irrational. If declared irrational a second comparison will be done after a short drive cycle.