The A/C clutch relay control circuit monitors the operation of the air conditioner. To do this, power is supplied to the A/C clutch to connect the A/C compressor. This circuit can be very complex, and it includes several modules, depending on the vehicle manufacturer.
The PCM (Powertrain Control Module) sets the DTC P0645 OBDII and activates the Check Engine light when a general malfunction is detected in the air conditioning relay control circuit.