1998 Buick Century Owner's Manual - Page 152

Ventilation System Adjust the direction of airflow by moving the vents. Ventilation Tips 0 Keep the hood and front air inlet free of ice, snow or any other obstruction, such as leaves. The heater and defroster will work far better, reducing the chance of fogging the inside of your windows. When you enter a vehicle in cold weather, adjust the mode knob or button to FLOOR and the fan to the highest speed for a few seconds before driving off. This helps clear the intake ductsof snow and moisture and reduces the chance of fogging the inside of your windows. Keep the air path under the front seats clear of objects. This helps air to circulate throughout your vehicle. 0 0 Your vehicle's flow-through ventilation system supplies outside air into vehicle when it is moving. Outside the air will also enter the vehicle when the air conditioning fan is running. If the airflow seems very low when the fan knob is turned to the highest setting regardless of the mode setting, your passenger compartment air filtermay need to be replaced. See "Maintenance Schedule" in the Index. 3-10