The idle air control valve on your 2000 Ford Focus provides the on-board computer with a way to control the idle speed of the engine. It is a simple, controlled vacuum leak that allows more air to bypass when a higher idle speed is needed, or less air to bypass when a lower idle speed is called for. The symptoms of a bad IAC valve include an engine that will not idle (IAC stuck closed), an engine that idles to high (IAC stuck open), and an engine that stumbles and runs rough when the a/c compressor comes on.
Unplug the electrical connector on the IAC valve located on the back of the throttle body near the firewall. No tools are needed to unplug the connector. Squeeze the release tab on the plug and pull to remove it.
Unbolt the two 8-mm attaching bolts attaching the IAC valve to the throttle body using an 8-mm socket and socket wrench from the socket set. Remove the IAC valve and gasket from the engine compartment.
Position the new valve and gasket in place on the throttle body, and bolt it to the throttle body using the 8-mm bolts and the socket and socket wrench. Avoid over-tightening as the throttle body is easily stripped out by over-tightening the fine-thread 8-mm bolts. Plug the electrical connector into the new valve, and push it on until it snaps into place.
Turn the ignition key to the run position but do not start it at this time. Unplug the electrical connector once again, and turn the key off.
Start the engine, and adjust the minimum air rate by turning the adjusting screw located on the throttle cable side of the throttle body until the idle speed is 650rpm plus/minus 50rpm. Turn the engine off, and plug the electrical connector back in--pushing until it clicks into place.