The front expansion valve is clogged.
Your heater will blow cold air only.
It doesn't really blow it out it just warms the cold air.
Probably your thermostat
There is a divertor controller under the dash that controls between heat and cold. Sometime the wireing harness works loose.
check the antifreeze level in the radiator, when a heater blows cool/cold air there is not enough antifreeze/water in the radiator.
you need to replace your heater core
it wont blow heat itll blow cold air
you are low on coolingliquid !
Probably because coolant level is low
heater core possibly plugged and leaking