The first possibility is the thermostat didn't work. To find this out, after the engine is warmed up u check the hoses going from the engine to the heater, they should be hot. if they are hot then the next reason would be a cable to the temp on the heater needs to be adjusted or the heater core is plugged inside or out.
Probably a bad thermostat.
Check the radiator for full coolant. Check the engines water pump and thermostat, as the engine coolant carries heat from the engine into the heater core.
Flush your radiator coolant and replace thermostat
bad thermostat or bad heat gasket
when running the heat-pump the air blowing from the heat ducts is not hot ( it's to cold for the heat-pump). you have a main furnace and it won't turn on ( is thermostat selected for this source), ( is the power switch for main furnace on ), ( is fuse for this furnace good). If these are good (check thermostat wiring at thermostat, and transformer).
Low coolant? Defective thermostat? Heat control cable not adjusted properly? Heater core plugged or airbound?
check your water or coolant level If coolant level is ok, replace the thermostat.
Vehicle takes an excessive time to attain normal operating temperature Heat not HOT with coolant full - thermostat not closing properly Vehicle heats up too soon - No heat with coolant full Coolant boils out of reservoir or top of radiator - thermostat not opening
A heat pump thermostat.
Car not blowing air or heat
heat and a/c barley blowing in back?