First thing you want to do is check the heater control valve on the engine side of the firewall attached to one or both of the heater hoses. Coolant must pass through it for the heater to work. If the valve is good, make sure you are getting flow through the heater core (you can run a garden hose through it) "blend door" is inside the heater box and is moved either by a small vacuum chamber (looks a lot like the vacuum advance mechanism on older chevy distributors). or it is moved by a cable. Whichever you have, it can be found on the heater box under the dashboard.
Easiest way to check them is to watch the heater box and with the engine running move the heater control Whichever type of actuator you have it should move the arm for the door when you do that. If you find one that does not work (for vacuum ones only) use a hand held vacuum pump and try to make it move. If it does, it's good if not, you either have a bad dash control or disconnected vacuum supply.
If cable type, find out why the cable isn't moving.
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Air
Temperature Control Blend Door Failure No Heat or no AC (dependes on
where door sticks)
In
all modern cars there is a tiny DC electric motor driven gear drive
that moves a plastic door that blends heated and cooled air from the
AC and the heater core, this is called (interestedly enough) the air
temperature control blend door actuator. The motor that moves this
door fails because the door starts sticking from warping, most common
complaint is no heat but it can be no AC as well. To replace these
parts (blend door and blend door actuator) you must remove the entire
dash from the car. Always have this problem confirmed by a dealer or
qualified repair shop as it is a very expensive repair.
Other causes of no heat in the cabin are engine thermostats that are stuck open (engine runs cold) or air bubbles in the cooling system.
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