The idle air control valve (sometimes called IACV is an ECU-controlled engine speed regulator which allows a metered amount of air to bypass the throttle body plate so that the engine maintains a smooth idle speed and doesn't stall out.
The ECU has the ideal engine idle speed for given situations stored in a program within it. The ECU compares the actual engine speed (information gathered by either the camshaft position sensor or the crankshaft position sensor, depending on the design) to the stored desired speed. If the speeds do not match, power is sent to the internal solenoid to open or close the valve. Opening the valve to let more air in will raise the motor speed, while closing it and reducing the air flow will have the opposite effect.
How an Idle Air Control Valve Works
In this early Cars Simplified video, Steve explains how an idle air control valve works, using a throttle body from a GM 3100 SFI V6 out of a 2001 Pontiac Grand Prix. This video was posted on April 23, 2016 and the intro song is "First Try" by Futures Passed.
Faulty Idle Air Control Valve Symptoms
The only time an idle air control valve is in use and serves any purpose is at idle, so an unstable idle, high idle, struggling while crawling forward, or just stalling out can be signs that it is not working properly. An OBD II Code P0505 may be presented when the IAC valve is malfunctioning, but the vehicle computer can't always detect the problem, so a lack of the P0505 code shouldn't be assumed to mean a good idle air control valve.
IACV Component Installation
Replacing an IACV starts with disconnecting the battery and removing components which may be in the way, such as the intake tubing. Remove the bolts holding the valve to the throttle body.
After the valve has been removed, the air flow ports must be inspected and cleaned to prevent immediate failure of the new IACV. These ports often get clogged up with carbon build-up which could resist the movement of the control valves where the valve body is inserted into the throttle body (as opposed to being encased in its own unit). The ports should be cleaned in general for proper engine performance. Compare the new valve to the old one to check for damage, new part accuracy, and potential required adjustments.
Installing the new valve should be the reverse of the removal steps. The final step for some vehicles is a re-learn procedure, which can be done by starting the vehicle and letting it run at idle until the operating temperature is reached.