I've had two Mazda's, a 97 323 and 98 626. On both these car's engines there was only a single IAC mounted on top of the throttle body, and no other device for adding air flow on the intake side.
However, on my 99 Nissan Almera with the GA16DE engine there are two IAC type devices mounted on one side of the throttle body, one called the
Auxiliary air control and the other called the
Fast idle control device:
A system diagram in the engine control manual shows both these as part of the idle air control system ( pg 12 ):
In addition, there is also have a Fast Idle Cam ( pg 55 ) on the throttle body which seems to directly open the main throttle plate:
Why put so many idle air control devices instead of just one like on the Mazda's I've seen. What is the purpose of each one?