Acura CL climate control

I've replaced the climate control circuit board 3 times now on my 97 Acura 3.0 CL. Is there something else that could cause the climate control to malfunction? Setting the temp to max or minimum will not cause either heat nor ac to activate (it either stays on heat or ac). I believe the control unit will not cause the duct gates to open or close according to temp conditions. All circuit boards were purchased used so it may be that their life-spans were nearly up when I got them.