'92 Lumina Heater Controls

My Lumina has push button controls for the heater and A/C . No matter which button I push the unit essentially stays on Defrost. Can't get anything to blow out of the side vents or floor vents. Fan works fine just can't select which vents the heat is to come out of. The air blowing on the window doesn't seem to be as hot as it should be. There is a slide bar that you move left or right to switch from cool to heat setting. The little lights work indicating which button has been pushed.
I saw in another thread that there might be a vacuum leak but I'm not sure if that applies to this unit. It seems to be a total electronic unit but I'm not sure.
Thanks for your help in advance.

Sounds like vacuum to me. These controls are electro-pneumatic, when you push a button to select it electronically activates a solenoid which in turn routes vacuum to the airgate next to the heater core. I'm willing to bet that you have either a vacuum leak somewhere along the way or a plugged vacuum line somehow. When there is no vacuum going to the control unit, it defaults to the defrost air pathway.

Check fuse & courtesy panel also. Fuse panel in glove box, courtesy panel under right side of dash.

Thanks guys!
I'm starting now with the fuses and then if no luck there I'll work my way through the vac system.
I appreciate your quick responses.
Thanks Again!

It's a vacuum leak- look underhood at where vacuum line crosses over top of serpentine belt at power steering pulley- they are usually just unplugged here by someone who bumped it- or forgot to plug it back together- or it can be worn through by rubbing in the pulley- even through the black plastic conduit....takes about 30 seconds to fix usually- just splice a piece of rubber hose over the broken vacuum line- you can cut off spare rubber hose from the windshield washer hose if you need it-- I have done several this way in the parking lot where I work.......

My Lumina has push button controls for the heater and A/C . No matter which button I push the unit essentially stays on Defrost. Can't get anything to blow out of the side vents or floor vents. Fan works fine just can't select which vents the heat is to come out of. The air blowing on the window doesn't seem to be as hot as it should be. There is a slide bar that you move left or right to switch from cool to heat setting. The little lights work indicating which button has been pushed.
I saw in another thread that there might be a vacuum leak but I'm not sure if that applies to this unit. It seems to be a total electronic unit but I'm not sure.
Thanks for your help in advance.

well if your not sure i guess it's about time to find out ......if you don't want to get dirty best let a professional handle it.........

Thanks everyone for your advice. I traced the problem to a broken vac. tube located under the battery. After a quick fix as suggested by brcidd it is now back up and running better then it has for a few years. Thanks again everyone!