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Windshield Defogging Operation
In AUTO mode, some airflow is directed to the defogger vents automatically in colder temperatures. With this button selected, all airflow is diverted to the defogger and side dash vents and fan speed is increased to rapidly clear the windshield and side windows. The A/C is also activated to improve defogging ability (except under very cold conditions). If selected, A/C Econ mode is canceled to improve defogging ability. To return to the previous mode, press the button a second time or press AUTO to restore automatic control. Other vent modes are operated separately.
[Mazda Millenia climate control - From 2002 Mazda Millenia owner's manual]