Could be a clogged heater core. Just because it doesn't leak doesn't mean it's working right.
you have an airlock
Heater core changed three years ago,I changed Thermostat Heater still blows cool air.
Your thermostat needs to be changed. It's about $10.00 for the part and simple to change. It's probably where the top radiator hose meets the enging block. Don't forget to boy Permatex gasket sealer and use it when you change the thermostat.
control fuse blows when you plug thermostat to base controlling a gas pack unit
you might want to check and see if the heater core is bypassed
That is normal, It is a thermostat
Bad Thermostat
Low coolant? Bad thermostat? Heater core airbound? Heater core restricted or plugged? Defective radiator cap not building sufficient pressure?
When my a/c system is low on refrigerant, it blows hot on drivers side, cold on passenger side of the dashboard. This is a 1997 Buick Ultra. Recharging the system solves the problem.
You might have bad actuator that opens and closes airways.
Thermostat is bad
I have changed heater core,water pump,thermostat, the fon works well but only blows cold air plus i have flushed the system.