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the fast idle thermo valve allows more air to enter the motor which raises the idle during during the first couple of minutes of operation. it has coolant lines running to it and as the water temp increases the spring inside gradually closes the valve untill its fully closed and your iacv takes over 100% of the idle adjustment process.

if you live in a warm climate just block it off. remove it, make a gasket out of a cardboard cereal box that blocks all the holes and reinstall it. reroute the coolant lines going to it also. you will not throw a code as there is no plug going to it.

if you live in a cold climate replace it. you'll have a surging idle or shitty idle untill you do as the valve is most likely stuck open. which in turn allows to much air into the motor once the car is warm and maxs out the duty cycle of the iacv causing the poor idle conditions.

get one from a junkyard and save yourself some money.
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