The throttle blade is underneath the air sensor plate, and controlled by the throttle linkage. It has a stop screw on it that allows a small bit of air to pass thru at idle. Just like an old carburator. The IAC adds the rest of the air, depending on need. If the throttle blade is too far open when the accelerator pedal is closed at idle, then the IAC valve cannot adjust the air flow down enough. With my IAC completely closed or blocked off, the engine idles at 500 RPM with the air sneaking past the barely open throttle blade.
then the IAC comes on and adds air to achieve the 600 RPM idle with the AC on and the tranny in Drive.
may not be it, but worth a quick check.

DG