The PCM monitors the A/C Refrigerant Pressure Sensor. The A/C Pressure Sensor is placed on the high pressure line. This signal indicates how much load the A/C compressor is putting on the engine and is one of the factors used by the PCM in order to determine the idle air control position for the idle speed. The signal is a voltage which is proportional to the A/C pressure from 0 to 5V. Low pressure produces a low voltage signal and high pressure a high-voltage signal.