Yes, those are some of the components; but, what are they doing, and how are they doing it, and are they doing in correctly, and in the correct amount or degree?
The questioning needs to move into the present tense. What is happening NOW, to what, for what, from what, why, ect.
wouldnt a scanner read if one or several of those components are faulty?
as youve said, one can not jump into the middle…you have posed a question about the integrity of the car’s idle control system,its components,how those works and thereby,knowing how they would not work.as for the present ‘what’,rpm goes down by turning the aircon on OR by shifting gears.why? so far,no one knows.
You can depend on a scan tool to a certain extent; but, the scan tool reports on what the engine computer sees and reports. There are things the scan tool doesn’t report on, such as fuel pressure (on Honda). It’s good to back up the scan tool data with other sources, such as a fuel/air tester, vacuum test gauge, fuel pressure test gauge, etc.