[Zzorro]

I'm troubleshooting my A/C / heater system. Symptoms:
-only get heat when it's turned all the way to high heat. No intermediate heat.
-AC does not blow cold. I know I need to recharge the system (or more maybe)
-I have inspected the "flap" and they are in good shape with the original covering sill on them and no holes. (Sent a small camera into the center duct to have a look). I thought for sure this was the issue based on my reading here and elsewhere, but it wasn't).

So? What else do I check to troubleshoot the heat? Does the AC need to work before I can confirm the heater issue? Are there sensors that need to be swapped? Or the controller?