2008 GMC Sierra 1500 Pickup Owner's Manual - Page 164

To avoid blowing cold air in cold weather, the system delays turning the fan on until warm air is available. The length of delay depends on the engine coolant temperature. Pressing the fan switch overrides this delay and changes the fan to a selected speed. Press this button to turn off the climate control system. Outside air still enters the vehicle, and is directed to the floor. This direction can be changed by pressing the mode button. Recirculation can be selected once you have selected vent or bi-level mode. The temperature can also be adjusted using either temperature button. If the air delivery mode or temperature settings are adjusted with the system off, the display illuminates briefly to show the settings and then returns to off. Press the on/off button or the up down arrows on the fan switch, the defrost button, AUTO button, or the air conditioning button to turn the system on when it is off. Manual Operation The air delivery mode or fan speed can be manually adjusted. DC (Fan): O(On/Off): These buttons let you manually adjust the fan speed. Press D to increase fan speed and C to decrease fan speed. Pressing either fan button while the system is off turns the system on. Pressing either fan button while in automatic control places the fan under manual control. The fan setting remains displayed and the AUTO light turns off. The air delivery mode remains under automatic control. HG (Mode): Press H and G to manually change the direction of the airflow in the vehicle. Repeatedly press either button until the desired mode appears on the display. Pressing either mode button while the system is off changes the air delivery mode without turning the system on. Pressing either mode button while in automatic control places the mode under manual control. The air delivery mode setting is displayed and the AUTO light turns off. The fan remains under automatic control. 3-28