The idle air control valve or IAC on a 2001 Toyota Corolla is on the throttle body. It is a magnetic rotary valve that blocks or exposes an air vent in the throttle body. The PCM or power control module sends a signal to the IAC that governs how much the valve opens or closes, keeping a constant rpm. The IAC contains a spring which is designed to retract the rotary valve when it is extended. The PCM extends the valve and the spring retracts it. The IAC on this model tends to allow oil to infiltrate into its body causing it to stick. It can't be cleaned or serviced; it must be replaced.
Start the engine and allow it to warm up to normal operating temperature.
Turn on the lights and the radio, the air conditioning and the fan, and set them to their highest settings. If you hear the engine rpm rise to compensate for the extra energy load, the IAC is working. If the rpm drops, you must replace the idle air control. Before determining this, perform the last test.
Shut the engine off. The plug on the IAC has three wires in line. As you are looking at the plug, the wire to the far right is the signal, the wire in the middle is the battery power and the one to the left is the ground.
Connect the voltmeter's red lead to the centre wire and the black lead to any good ground. Turn the key on and check for battery voltage. If the voltmeter displays 12 volts, the wiring and plug are good. Replace the IAC. If the voltage is not present, check the fuses and if they are functional, check the wiring from the centre wire to the fuse box for a short or open in the wire.