I have a 93 lesabre. The air and heat controller has a mind of it's own. When a light is lit on the controller you have control of the functions. When no light is lit all you have control of is the temperature slide. I have replaced the controller and the new one (used)does the same thing. Does the air/heat have it's own computer and if it does...where is it located? thanks, Captain Jim

Your vehicle use's three module's that make up the body control system .
Body Control System Operation
Each of the 3 body control modules integrate a number of functional systems under the control of a single module. Each of the modules is connected to the Class 2 serial data line. Many control signals are implemented by Class 2 messages.
Dash Integration Module (DIM)
The DIM functions include the following functions:
• Power moding control (ignition switch states) over Class 2 serial data line
• Control of headlights and exterior lamps
• Interior lamps incandescent dimming
• Steering wheel controls
• Horn relay control for theft activation
• Lamps On signal with wiper/washer
• Low side temperature for HVAC compressor
• Cooling fan control (power - train cooling)
• Incandescent dimming of dash bulbs control
• Real time clock time keeper
• Battery voltage sense and load management
• Reverse lockout solenoid control
Instrument Panel Module (IPM)
The IPM functions include the following functions:
• Front HVAC air delivery and temperature controls
• Ambient light sensor input and twilight delay input for headlights control
• IP dimmer switch input
• Traction control switch input
• Interior lamps switch input
• Key in ignition switch input from the ignition switch
• Rear compartment lid release switch input
• Ignition switch headlight control
Rear Integration Module (RIM)
The RIM functions include the following functions:
• Heated seat controls
• Rear defogger control
• Park relay controls
• Various controls for the interior lamps
• Reverse control for the backup lamps
• Rear compartment lid release controls
• Retained Accessory Power (RAP) control
• Ajar switch and tamper switch inputs from the rear compartment lid
• Vehicle leveling
Your best bet , take your vehicle to a qualified repair shop , before you cause unnecessary problem's an repair's .

vacuum hose to vent door could be off or busted, problem in climate control. replace idle air control valve to bring in down

Do some additional research, but they sell a lot of Climate Controls on ebay. 1995-1999 Buick Lesabre, Riviera, Park Ave Digital Climate Control P/N 16181974 (Fits: Buick LeSabre)

Control head is failing

If you can get the compressor to shut off with vent/floor settings, I imagine it's still using defrost in auto mode. Never liked ACC. :)

Your A/C Heater Control needs to be replaced.

check to see if both hoses are hot going to your heater core if they are then core is fine.That would point to your temperature sensor.

check the heater control valve. sounds to me like the dash control unit that opens and closes the vents may not be functioning properly

Change your Thottle positioning Sensor. My 93 lesabre did the same thing and i changed the TPS and it stopped.

this system has self testing capabilities, have any GM dealer test it for codes and proper function, If I had to guess I would say the climate control head is defective.