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.
The BCM internally monitors the voltage on the signal circuit, which is converted into a temperature reading. The BCM broadcasts the AAT Sensor signal over the Can Bus. The Powertrain Control Module (PCM) acquires the AAT Sensor signal from the Can Bus. 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 on a cold start after a cold soak condition was present on the previous key off cycle. Following a start to run delay time, the PCM compares the signals from 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 Sensor signal is declared as irrational. If declared irrational a second comparison will be done after a short drive cycle.
While circuit high and low faults are monitored by both the PCM and BCM, only the PCM performs a rationality diagnostic of the AAT Sensor signal.
If a repair is performed to the AAT Sensor wiring, or a sensor is replaced to correct and active DTC, the vehicle must be driven for a minimum of 5 minutes above 15 mph to update the AAT temp signal before the DTC will go stored and can be erased.