Normal operation: 
- Recirc: Vacuum pressure pulls opens inside air source / Servo Closes fresh air source
- Fresh: Vacuum releases & spring loaded inside air source closes / Servo Opens fresh air source
The problem is that the electric servo could die anywhere - fresh air source open, closed, or somewhere in between. So the only real thing we are guaranteed to know is if the vacuum fails.

If vacuum fails, and servo still operates: 
- Fresh air selection: servo opens fresh source, inside air source closed. NORMAL
- Recirc selection: servo closes fresh source and inside air remains closed. NO AIR FLOW.

If vacuum works and servo fails with fresh air closed:
- Fresh air selection: servo remains closed, inside air closed. NO AIR FLOW.
- Recirc selection: servo closed, inside air opens. NORMAL.

If vacuum works and servo fails with fresh air open:
- Fresh air selection: servo remains open, inside air closed. NORMAL.
- Recirc selection: servo remains open, inside air opens. MAY APPEAR NORMAL, but can't shut off outside air.

So to sum up:

- No air flow/Wheeze on recirc - you have a vacuum leak somewhere: 
- No air flow/Wheeze on fresh - your fresh air electric servo has died in the closed position.

To verify electric servo: 
- Remove black plastic trim under front hood nearest to the base of windshield and locate center electric servo. (There are 3 servos on a 993, one to open/close footwell output, one to open close windshield output, one to open close fresh air input.) 964's have 2 more to control temp mixers. 993 temp mixer servos are in cabin under door sill trim area. 
- Operating the corresponding a/c control should show servos moving back and forth.

To verify vacuum -
- Main vacuum "source" from engine routes to same location as the 3 servos, into an electric solenoid. When the recirc button is pressed, the electric solenoid opens the valve, passing vacuum pressure to output. With engine idling: 
- Disconnect vacuum source on solenoid to verify vacuum pressure. If not present, problem is vacuum leak in engine area. (Good luck) 
- If vacuum present, press recirc, disconnect output side of solenoid and verify vacuum pressure. 
- If not, solenoid not getting signal or failed. 
- If present, look under dash, under ccu for rubber elbow connecting vacuum line to flap mechanism - it would most likely be unplugged due to stereo installer or other inside work bumping it free.