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Most likely it is the heater control valve that shifts air from blowing across your heater core to your air conditioner core.
If your fan works and your car is fairly new..meaning your heater core isn't dirty and full of craap...then it is the climate cotrol valve.
If you are lucky, it might be a thermostat that is bad and blocking hot water from entering your heater core. Bottom line, if hot water doesn't circulate thru that little heater core, you won't get heat.
You might be able to look under the hood and grab the hot water lines that go thru the firewall and feed your heater core. If they feel cold to the touch after engine has warmed up, then you aren't getting circulating water to the heater core. If they feel nice and warm, then the heater core is fine, but your climate control valve has closed off the fan that blows across the core to disperse heat into the cabin
could have air lock or heater core plugging up. heater cores should be back flushed.when car is hot touch one heater hose at a time both should be fairly hot if both hot then the problem is temp control.
when you put the heater on hot and the blower on full and the car is running at idle, the coolant in the heater matrix is cooled quickly,as when the radiator fan comes on to cool the engine, have you tried raising the revs and checking the heater to see if it gets hotter. If this does not happen you could have air lock in the cooling system and may need bleeding properly.
Some have heater control valves at the heater inlet by the firewall. Either vacuum or cable operated, they open the water flow to let hot water into the heater core. See if its working by checking for vacuum at the hose when the heater control is on hot. Dealers and part stores usually have them in stock.
First make sure you have the radiator full of antifreeze also check overflow to make sure its at proper level . then let car warm up and feel hoses that go to firewall tword heater core and see if one of them are warm if so you have a blocked water control valve or its got a vacuum line off going to it .. Thanks Jerry holler if this is not it we will go from there.
The heater in your car depends upon a flow of hot coolant going through the heater core in order to get hot air into the car. If the heater core is partially plugged, it will restrict the flow. If you have a head gasket that is putting exhaust into the cooling system, it will eventually get into the heater core. When this happens there is no coolant in the heater core to make the air warm. If the water pump vanes have corroded away the result will be very poor coolant circulation causing the heater to not blow hot air. If your car is equipped with a heater control valve that is not functioning properly, the coolant flow will be restricted. One or a combination of these factors is likely the culprit in your case.
To attempt to clean the core, you must flush the system multiple times. If this does not work, the core cannot be cleaned and must be replaced.
Warm up vehicle. Then, from the engine compartment, grip each heater hose to check if they are HOT. Not warm HOT. If they are both hot, then the BLEND DOOR is stuck to the cold position. Access this thru glove box area.
If they are not HOT, then check radiator hoses for proper heat. If radiator hoses are not HOT then thermostat is stuck OPEN.
If they are HOT then there may be a restriction in circulation to the heater hoses and heater core.