Q: How does the Idle Air Control (IAC) valve regulate engine idle speed and How to check and replace it on Nissan Quest?A: The Idle Control Valve controls the air that is freed from the throttle valve affecting the engine idle speed. It is installed on the upper intake plenum and works in response to the voltage signals from the PCM that will make the valve to either move in or out to control the intake of air. To perform the initial check, the PCM's voltage signal needs to be ascertained; do this when the ignition key is turned On and while probing the Idle Control Valve electrical connector with the voltmeter will give a 10.5 to 12.5 volts' reading. If the voltage is okay, check the resistance at the Idle Control Valve terminals, and it will be around 10 ohms; otherwise, the valve should be replaced. Further, perform an internal short circuit check by measuring the resistance from any terminal to the Idle Control Valve body, it should be 10000 or more. If it is less, get the valve replaced; Before reassembling, clean the pintle if there is carbon deposit after removing the valve of the engine. For replacement, disconnect the electrical connector located on the Idle Control Valve as well as the FIC solenoid, remove the position of the coolant hose, and finally, remove the housing of the Idle Control Valve/FIC from the upper intake manifold. Remove the Idle Control Valve out of the housing and inspect for damage of the O-ring and clean the sealing groove. Dip a new O-ring in some engine oil and then fit the Idle Control Valve back in place and tighten the respective screws while allowing the removal sequence to guide you.