The AAV (Auxiliary Air Valve), whose real name is "Extra Air Valve", can be assessed by general observation, a test, a peek and a poke.  It is an aluminum housing bolted with allen head screws to the water rail (the rail is part number EAC2915 in Europe or EAC3077 everywhere else) directly underneath the ignition coil bolt boss.  There is an electrical connector on the left side and a large hose connected to the bottom.

The electrical connector is one of the 2 conductor Bosch-style FI plugs.
Walt Chrush

Observation

When the AAV is working correctly, one observes that the cold idle is higher than the hot idle.  It drops to set hot idle (as specified on the deacal underhood - usually 750 RPM) as the engine warms up.

Test

One can test by clamping the upper air hose to the AAV with the engine idling.  While cold, the rpms should drop.  When hot there should be no change.

Peek

One can disconnect the air hose and peek inside the AAV.  The slide should be half open when cold, and closed when hot.

Poke

One can poke with a thin screwdriver.  The slide should move against spring loading, and reset when released.
Frank Andersen

Inside the AAV.  The self-tapping screw was jammed against the spring so you can see the teapot-shaped bypass hole.

A view of the bimetallic strip with the heating coil around it.

The AAV is constructed much like an old choke system.  The heating element is composed of both a bimetallic spring and a heating coil.  Once the engine is warm enough the spring maintains its closed position without any assistance from the heating coil, so the ECU opens the circuit to the heating element.
Walt Chrush

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