I am fully aware of the heater door problems but do not think I have that problem...hopefully. when chsnging from vents to floor/defrost it does nothing...When changing to anything it does nothing but come out the 4 face vents. Could there be a loose vacuum. I know the t over the master cylinder can sometimes be an issue..it looks brittle but still in tact...I also noticed the heat wheb on 3 or 4 doesnt come out as powerfully as it used to...any help is better than none it just hit -6 here

Thank you

 

The HVAC vents are controlled by vacuum actuators so check the vacuum lines under the hood, left side, up by brake booster.
If all lines are good, then could be the rotary switch in the HVAC panel, these do leak and need replacement. Is there a hissing noise coming from the HVAC panel when the engine is running? If so, then replace control. RockAuto is my suggestion.

 

Knock on wood no hissing noises that are noticeable...wheb I drive home from work ill leave rhe radio off and see if I hear it hissing. With the vacuum lines...would I have to take apart the dash to get to them?

 

same issue in the 2001 I just got. Many of the lines are brittle, some are the wrong size, and with this cold I am just glad to have heat in the thing. Cant wait for warmer weather to really sort this out. Keeping my eye on this thread to see what happens

 

Check the vacuum lines under the hood first; could be that there is no vacuum getting to the HVAC control.
Actually if the lines under the hood have never been replaced very likely this is the problem.

 

Thanks, I just developed the same issue. Will be checking those lines.

 

I didn't touch anything yet, and it works again lol...I checked lines some look brittle but none appear or sound to be leaking. Hopefully winter will be over sooner than later than all I'll need is the 4 vents for the ac haha

 

If when any of the vacuum hoses are touched and black residue comes off onto your hand, replace that hose; even though a split or hole cannot be seen, the hose has deteriorated and is porous and leaking.

Rubber shrinks when cold so it may do a bit of self sealing and why it could be working. Same for the rotary vent position control....the rubber could self seal just enough to work for awhile.

I did learn a trick about the rotary vacuum switch in another car I have....put one or two drops of light oil (3in1) into the switch to get it to seal again. Worked on my other car...have never tried this on the Blazer since it is working fine.

I also re-read original posting; were you saying that the blower fan is not working as well as it use to? This could be a dirty blower resistor connection - not fun to get to, or dirty switch in the HVAC control panel...or just the blower motor going out.

-6°....no way would I be out there.....have lived far too long in the desert southwest to be in low temps like that anymore. Give me 115° over -6° any day.

 

It was the "polar vortex" haha...I was also talking to one of my customers who has an 03 trailblazer...he said it could also be cause by a clogged heater core. Is that a possibility?