An IAC (idle air control) motor is designed to adjust the engine idle RPM speed by opening and closing an air bypass passage inside the throttle body. The cars computer or PCM (powertrain control module) receives information from various sensors and will output signals to adjust the idle air control motor in or out to adjust engine idle speed by controlling engine idle air. An idle air control motor can fail one of two ways, either the motor short circuits and stops working or the motor will develop high resistance and cause the idle air control motor to react slowly, either failure can cause the engine to stall at idle. When a trouble code scan is performed it sometimes wont always detect a failed or weak idle air control motor.
To check the idle air control motor remove the unit, with the wires connected turn the key to the on position without starting the engine, the idle air control should move in or out. If the idle air control motor does nothing it has probably failed, replace it with a new unit and recheck system. Note: while the idle air control motor is removed clean (use aerosol carburetor cleaner) the passages the idle air control uses to control idle air speed, also inspect the idle air control for a build-up on the seating (pointed) end and clean as necessary.