For 1990-2009 cars
A/C Relay Command: The parameter will indicate on whenever the A/C idle-up signal is received by the powertrain control module (PCM) from the A/C compressor control module. This will allow the PCM to adjust the engine idle to maintain acceptable engine performance.
ECT Sensor: The scan tool range is -40°C to +205°C (-40°F to +401°F). The engine coolant temperature (ECT) sensor is a thermistor which changes internal resistance as temperature changes. When the sensor is cold (internal resistance high), the PCM monitors a high signal voltage and interprets the voltage as a cold engine. As the sensor warms (internal resistance decreases), the voltage signal decreases and the PCM interprets the lower voltage as a warm engine.
Fan Request: The electric engine coolant fan is only on vehicles equipped with air conditioning. The scan tool displays on whenever the A/C is engaged or whenever the PCM commands the condenser fan on. The PCM commands the fan on for A/C operation and when an overheating condition exists (ECT 110°C (230°F) or higher).