'01 GMC, the controls have the Auto feature. I only get heat if I max out the temp to that final click. Anything lower than that and it blows cold. That's with the compressor on. I have to try it with the compressor off. But open to ideas.

Move controls (and truck) to Canada.

How's that?

(my best asian accent) You funny guy, you be here all week? I did offer it to Fund Razor in a package deal though, that'd fix it! Of course, being here in FL I never would've noticed if I hadn't left the windows down during a rainstorm...ugh

Thanks for the NAPA tip also; for some reason they're just at the bottom of my list as a source. But there's a small one close by that actually advertises they do industrial hose.

Have you checked to see if the CPU has thrown a code? My '06 Duramax w/auto controls would not engage the compressor for the A/C. Turns out it had thrown a U1300 code which was low voltage, and it put the auto A/C unit into "safe mode".

If you do not have access to a code reader and want to try a cheap fix first, you can try to disconnect the battery for 15-20 mins, a tech at the Stealership told me that it would reset all of the CPU's in the vehicle and clear most codes. Cheap and easy try, and if it don't work you are only out for your time.

No codes. Problem happens with or without A/C compressor so I'm thinking I need to educate myself on how the "mixer" works? On my temp selection knob, there's a definitive "knotch" that selects MAX Heat. It only starts blowing hot at that max setting.

So just how does an auto A/C system adjust the air temp?

I never run the A/C for heat. Its not needed and will only wear it down faster.

a/c runs on defrost so the seals stay lubricated.