Theory Of Operation
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The Engine Coolant Temperature (ECT) Sensor rationality diagnostic looks at the outputs of three temperature sensors and compares them under cold start conditions. Following a start to run delay time the outputs of the Ambient Air Temperature (AAT) Sensor, Engine Coolant Temperature (ECT) Sensor, and Intake Air Temperature (IAT) Sensor will be compared. If the IAT and AAT Sensors agree and the coolant temperature does not agree, the ECT Sensor is declared as irrational. If declared irrational a second comparison will be done after a short drive cycle.