The idle sensor is also called an idle control valve or an idle air control. It is normally found on the throttle body of the engine on a vehicle.
Engine idle speed is computer controlled by signals to the idle control valve mounted on the throttle body. The IAC valve regulates idle speed by moving a pintle valve in and out, which varies airflow through a passage bypassing the throttle valve. The more air that is allowed to bypass the throttle valve, the faster the idle speed. At engine speeds above idle, the idle control valve is not used.
Internal air ports regulate the volume of air that passes through the valve. When an idle sensor fails, it can cause the engine to idle erratically or stall. Sometimes a weak sensor causes problems without even triggering the check engine light.