I am also having a problem with the heater controls on my 2005 Trailblazer. I noticed a fwe months ago that the "5" setting would cut in and out. Eventually that setting quit all together. Then the "4" setting quit as well... It's getting hard to keep the truck warm with 3 being the maximum fan speed! I think it is just the selector switch that has gone bad, but I am having some other wierd electrical issues (see previous post). Can I change out the controls farly easily? It can't be as hard as removing the dash to change the heater core on my old 2000, but I really don't want to repeat that adventure again anyway! Thanks.

 

I would check the blower resistor.

 

Thank You!!! After reading about the resistor I believe that is exactly what the problem is. There are way too many parts on vehicles these days!

 

The blower motor resistor is another a known problem on these trucks. Its a good place to start. I have replaced quite a few for friends & family over the last few years. It is easily accessible from under the passenger side dash.