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The HVAC doors that control the airflow are activated by small vacuum motors. The system is designed to fail safe; this means that the system will fail in full defrost mode. So, knowing this and the fact that you have heater but no A/C or defrost you should assume the system has a failed control head (the part that you select functions with with button or lever). Or the doors are stuck due to an warped duct system (usually high heat related).

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โˆ™ 2011-09-13 01:33:33
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Q: How do you figure out why the heater and blower work but the ac and defroster do not?
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