I've mentioned it a couple times. Check the changeover valves located under the panel to the right of the radio. Test each one for electrical continuity. They burn out over time, leaving the heat/cool flaps in their default positions. You should check them first before trouble-shooting the vac elements. Two of mine were burned out, and caused most of my ACC problems (the other culprit being a bad sampler fan).

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma