Connected to the engine's primary air intake valve (the Throttle Body, or TB), the Idle Air Control (IAC) valve is responsible for making minor adjustments to engine airflow. While the IAC valve's effect is fairly minimal while the TB valve is open, many engines rely on the IAC for idle or low-speed operation. IAC valve mechanisms often begin to malfunction after a few hundred thousand miles, but can become dirty and clogged in far less time.
M/F IAC-related no-start conditions usually only occur on older vehicles with fully electronic (drive-by-wire, DBW) throttles. DBW systems (like those used on the Chevrolet Corvette and Toyota Prius) use a switch on the gas pedal to control a servo on the TB. Newer drive-by-wire cars don't have an IAC, but older DBW-equipped models depend entirely on the IAC just to start.
M/F Most IAC failures manifest themselves as a start-and-stall condition. Generally speaking, the engine will catch and run for a second or two, stutter and die. IAC-related start-and-stop conditions can be easily differentiated from fuel-related issues; during IAC-related stalling, applying throttle in light pulses may prolong engine failure, but will not prevent it. Applying throttle while facing fuel-related failure will result in immediate stalling.
M/F When rapidly decelerating, the driver lifts his foot from the throttle pedal and applies the brake. Once the driver's foot is clear of the throttle, the engine's TB valve completely shuts. In the case of a malfunctioning IAC, rapidly shutting the throttle pedal will starve the engine of air, resulting in a stall either while decelerating or after coming to a stop.
M/F A very cold engine needs more fuel than a warm engine, which is important to know while diagnosing the problem. An obstructed or stuck-shut IAC will result in a rich air-fuel ratio at idle, so the engine will start very easily while cold and then die suddenly after a slight puff of black tail-pipe smoke. A stuck-open IAC might start fine while warm and may continue to run if the engine reaches high-enough RPM. Because most IACs break while in the open position, a "stuck-shut" diagnosis is usually indicative of one that's simply clogged.
M/F An intermittent stalling condition is one in which the vehicle might start fine, act as though it's going to die, idle smoothly and then die completely. Alternatively, the vehicle may run fine most of the time, but stall periodically and then restart with a bit of difficulty. Intermittent stalling almost always indicates a dirty or clogged IAC.