i very much doubt its the thermostat, if it wasnt opening you would certainly know about it, as the engine would quickly overheat. however, if it was stuck open, your engine wouldnt get upto temp. thermostat allows coolant to circulate around the engine once open, nothing to do with the heater matrix. when the thermostat is closed, the water inside the engine and the heating system is allowed to heat up, which is why the heaters should blow warm before the thermostat opens. once to the correct temp, thermostat opens and coolant flows through the engine.
within the dash, there are control flaps, which basically mix warm and cold air so the temp on the atc unit is near enough what is coming out of the blowers. sounds like either the temp sensors is broken or reading wrong, the flap which controls warm is stuck closed/partially open, or the flap which controls cold air is stuck fully open, if there isnt any air in the heating system.
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