Buick Lucerne 2007 Owner's Manual page 192

Defogging and Defrosting
Fog on the inside of windows is a result of high
humidity (moisture) condensing on the cool window
glass. This can be minimized if the climate control
system is used properly. There are two modes to
clear fog from your windshield, defog and defrost.
W
(Floor/Defog): Use this mode to clear the
windows of fog or moisture and to warm the
passengers. Turning the control to this mode
directs about half of the air to the floor outlets and
the remaining air is directed to the side window
outlets and to the windshield. When you select
this mode, the system turns off recirculation and
runs the air-conditioning compressor unless the
outside temperature is at or below freezing. The
recirculation mode cannot be selected while in the
floor/defog mode.
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(Defrost): Use this mode to remove fog or
frost from the windshield more quickly. Turning the
control to this mode directs most of the air to
the windshield and the side window outlets. In this
mode, the system will automatically turn off
recirculation and run the air-conditioning
compressor, unless the outside temperature is at
or below freezing. Recirculation cannot be
selected while in the defrost mode. Do not drive
the vehicle until all the windows are clear.
Rear Window Defogger
The rear window defogger uses a warming grid to
remove fog or frost from the rear window.
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(Rear Defogger): Press this button to turn
the rear window defogger on or off. An indicator
light below the button will come on to let you know
that the rear window defogger is activated.
Be sure to clear as much snow from the rear
window as possible.