2001 GMC Sierra 1500 Pickup Owner's Manual - Page 192

The auto recirculation feature will activate when the following are true: D the air conditioning system is activated, D the OUTSIDE AIR button is on, and D the vehicle load is high due to city traffic, extended idling or hot weather. The system will automatically switch to the recirculation mode to lower the system load and improve interior comfort. The OUTSIDE AIR and RECIRCULATION button LEDs will not change. The system will return to the OUTSIDE AIR function when the system load is reduced. A/C: Press this button to turn the air conditioning on and off. The system will cool and dehumidify the air inside the vehicle when the A/C light is on. Air conditioning will be automatically disabled when the outside temperature drops below a level at which air conditioning is ineffective. Electronic Climate Control System (If Equipped) Fan Control The knob on the left side of the electronic climate control panel operates the fan speed. To manually increase airflow, move the knob clockwise. To manually decrease airflow, move it counterclockwise. OFF: If the knob is in OFF, outside air will still enter the vehicle, and will be directed based on the position of the mode knob. 3-5