A/C relay which is controlled by ECM supplies and cut electrical power to A/C compressor. ECM activates or deactivates A/C relay based on inputted signal such as A/C switch signal and A/C pressure switch signal. As controlling A/C relay, ECM 1)turns OFF A/C compressor at rapid acceleration to retain sufficient capacity for acceleration, 2)actively performs idle-up function to cope with the change of engine load which happens at A/C compressor operation.

P0646 is set when '0A' is detected in A/C relay control circuit for more than 1 sec.. This code is due to open or short to ground in A/C relay control circuit or internal open in relay component.