I had this problem, after pulling the intake for a clean-up. Seems someone had not reattached a small vacuum line near the firewall :rolleyes:

HVAC defaults to defrost in case of insufficient vacuum...might want to check for loose or cracked hoses under the hood...

Marco Tatta
98.5 SE MTX, Duratec
EGR block, fog light
fix, custom shift boot, monsterflow intake, Ecotek valve
Quasi dual cardoctor exhaust.
Hacksaw short shift, Momo race "s" carbon knob, ghetto rear strut bar, 16 inch cougar wheels with 225/50/16 kumho 712s