2004 Tahoe that started with no heat in rear. Checked all hoses and they get hot all the way to back. Replaced rear blend door actuator and worked for a little while. Tonight, took a 90 minute trip and it blew ice cold until suddenly it started blowing heat 45 minutes into trip but then went back to cold after about 10 minutes. Actuator is electric not vacuum. Could it be in the controls where you adjust the temp?
Make sure your fan speeds are working, if not a variable resistor is in order.
I don’t see how the fan speed would affect the temperature of the heat coming out. If this is electronically controlled, I think you have a problem with the climate control system. When it blows cold air, try turning the temp control all the way to cold slowly, then turning it back up to all the way hot. I suspect that you have a bad climate control unit, or bad ambient temp sensor that is fooling the system.
There may be a button sequence you can press to retrieve trouble codes from the unit or to reset it. I would Google for it. On my Mopar, you press and hold 3 specific buttons simultaneously for diagnostic codes and resetting it. I have no idea what it would be on your GM vehicle though.
Could be that it needs calibrated with a tech 2.
It could be the control but it also could be low coolant or air in the coolant. I had a very similar problem in an Envoy.
Its hard to say if your issue stems from the control valve or from an Air pocket in your cooling system. If your control valve is electric and not manual…try to listen carefully with the engine off key on and go from hot to cold…listen for the motor that controls the valve…can you hear it? If it is mechanical then you can check the back of the heater control by taking it off to see if the cable actually works the valve…
Aside from that you move to air in the system…but if you havent had the system low or added coolant this is unlikely.