If that's not it, it's entirely possible the control is bad. Both symptoms - vent position AND temperature problems - points to it.
The middle actuator runs the temperature control. On a 95-up it's roughly behind the ashtray; you can see it if you hinge the glovebox down.
Also check that the hoses going to/from the heater core both are hot.If one is cold and one is hot then there is blockage.Both should be hot.
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The blend door actuator or the entire switch itself. Any of the one can be issue.Before you go out and buy anything,Try this, get under the dash above the transmission hump and find the vent selection door and make sure that it isnt stuck. In some cases ,some users had somehow popped past the stopping point and wouldnt come back down. it getss stuck on full blast hot!! There actual temperature blend door is located behind the dash and thats what controls hot and cold. In most of the cases this checks out to be problem. You have to force the door back down into position and it works. The problem is that when you turned on the a/c it would only come out of the defrost, pretty much same thing as you just a/c not heat. Question- does the temperature control work, can you switch from hot to cold?.If that is not the case then blend door actuator motor has to be replaced.-----------
well the face/feet/defrost door is the one above the transmission hump. Its not too bad just get comfortable under the dash and look at it for a minute and you'll figure that one out. Now the hot/cold problem is a pain, i went and bought a used switch cluster from a salvage and ended up not using it. the blend door actuator is in the WORST spot that GM could ever think of, i hope you have small hands and are very easy going or else you will be throwing things and cussing whoever designed this crap, like I was!! It is located at the top, passenger side of the box that houses the heater core.
laying under the driver side the actuator motor RIGHT ABOVE the gas pedal is what controls the feet/face/defrost......It moves from the 3 O'clock to 9 O' Clock positions.... 3 O' clock being the defrost only and 9 O' clock being face only.
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