Theory Of Operation

The Ambient Air Temperature (AAT) Sensor rationality diagnostic runs under cold start conditions, following a start to run delay time. The diagnostic monitor compares the outputs of the AAT, Engine Coolant Temperature (ECT) and Intake Air Temperature (IAT) sensors. If the ECT and IAT Sensors agree, but the AAT Sensor does not agree, then the AAT Sensor is declared as irrational. If declared irrational a second comparison will be done after a short drive cycle.