It sounds like the control unit is going bad. Replace that whole panel there where the knobs for heat and air are and it should be fixed.
Heater core changed three years ago,I changed Thermostat Heater still blows cool air.
Check antifreeze level, then thermostat next, heater core last
possibly thermostat stuck in the open position?
Try your thermostat
thermostat stuck
some possibilities are. low coolant level, open thermostat, pluged heater core,
Low coolant? Bad thermostat? Heater core plugged,restricted or airbound? Heater control cable or valve not working?
It's either the thermostat or the heater controls.
You probably have a bad thermostat.
It sound like the thermostat is broken
check the thermostat dude may need replacing
Thermostat stuck open, heater core clogged, or the heater temperture control vlalve is defective.