probably you have a disconnected vacuum line. that line runs from the passenger side of the intake to the firewall and across the firewall and through the firwewall in fron of the drivers door by the windshield wiper motor. it then runs under the steerring column and joins some mechanisms above the accelorator pedal. when you find it and join it your car will run better too.
If defrost is on it could be your compressor going bad. See what happens when you leave off defrost or disconnect your compressor. It only controls defrost and air conditioning. I had the same problem
check the fan relay, it could be stuck on, otherwise make sure the heater controls are not set to defrost, and the ac is off, either of these things will activate the auxiliary fan relay.
Usually take the car to the nearest Subaru dealer and have then fix it. They invented it, they should fix it.
You have a vacuum line disconnected probably under the hood. The HVAC controls are operated by vacuum and when there is no vacuum it system goes into defrost mode.
most likely your heater controls run via vacuum lines. cracked or broken lines could cause this.
That means either you have air in your heater core or your heater core is being bypassed
I had a Chevy van of this vintage and I found a vacum hose leak in the controls that would make the heater/defrost door default too defrost open. lisining carefully under dash or under hood may pinpoint leak. I had a Chevy van of this vintage and I found a vacum hose leak in the controls that would make the heater/defrost door default too defrost open. lisining carefully under dash or under hood may pinpoint leak.
Check that the linkage that controls the damper is connected and working. The linkage is just above the gas pedal.
Many of these controls are Vacuum operated. You may have a problem where the "Main" Vacuum line has broken. See if you can locate this line by using your manual and make sure that it is intact. TommyTrouble
Push the rear window defrost button , located with your heater controls , that also heats your exterior mirrors
Don't know what vehicle you have but most HVAC controls operate from a vacuum supply from the engine. I would start with checking the vacuum supply connections under the hood. They could be cracked or disconnected.
Many cars use engine vacuum to operate HVAC controls. If vacuum is cut off the system defaults to the defrost mode. Check the HVAC vacuum supply hose under the hood leading from the engine. It may be disconnected or damaged.