The blend doors behind dash are controlled by vacuum from the engine that runs to the doors through a diaphragm. The defrost/floor position is the default position that the blend doors automatically go to if there is a malfunction. Usually when that happens it is caused loss of vacuum. You will have to inspect all vacuum hoses and HVAC system for any signs of leaking vacuum hose.
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Monday, March 30th, 2009 AT 10:17 PM