I have a little problem. My heat and A/C does not change, no matter what I set the control to. Luckily, it is stuck on a medium heat setting - but as soon as the temperature outside gets warmer, I am screwed.

Does anyone have ideas on what I can check?

 

I have no idea. But I think your heater core may be clogged or something.

 

I would say that or a broken blend door. But I would flush the heater core first, it is a cheeper option.

 

How would a clogged heater core cause medium heat on panel with thermostat set full cool? Clogged or not, the blend door should isolate the heater core.