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2001 Ford F150 heater/defrost won’t work

My F150 heater and defrost isn’t working. The fan works, ac works. What could be the problem?

The plenum door usually operated by a vacuum line under the hood is disconnected perhaps. Turn everything of and start it with the hodd yup anf listen for vacuum.air suckibg. But t is 2001 so who knows, check fuses just in case. Are the heater hoses going through firewall hot after driving a half hour?

How exactly does it not work? Does it blow cold air? Could be a stuck open thermostat, an air pocket in the cooling system, or the blend door as mentioned above. Does the engine temp gauge appear to be in the position it normally is when the engine is warmed up?