A failing or bad Idle Air Control Valve can cause problems such as engine stalling, rough idle, irregular idle speed, cause the cars engine to die suddenly and even cause the check engine light to come on.
An engine Idle Air Control Valve is there to open the throttle to allow a bit more air when needed and close when it doesn't need more air.
When it doesn't work your engine may not idle properly or will stall when it's idling and you car may also be hard to start due to a bad idle air control valve.