Heater troubles

My heater has decided I need to be cooked or something. No matter where you put the temp knob,( my controls have the auto setting for fan speed and air distribution), even with the a/c on the only thing that changes is the fan speed the heat stays full on to the point she will burn your hand if you hold it over the defroster vent. Is the problem a temp sensor or the air door controllers? In the past I cleaned a bunch of lint out of that little tube with the small fan inside with what looks like a temp sensor that mounts just inside of the front of the dash behind those 3 or 4 horizontal slits and that seemed to cure it but now it doesn't help.

 

05 Volvo vnl
I didn't think there was a heater control valve as in turning on and off the flow of coolant.

 

I stand corrected , There is a heater control valve and now I have to figure out how to check and replace it without getting coolant all over.

 

That was indeed the problem. The coolant control valve was bad. $156 in parts and 2 hours later and it works again. If you clamp off the coolant lines under the hood leading to the heater core you don't need to drain any coolant from the radiator.