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try running the ac as well as the heater. The ac removes the humidity (water vapour) in the air in the car which is causing the fogging up . Hot moist air on glass creates fog ( same as clouds). You will get a comfortable temperature inside without the fog
You are probably going to be at the mercy of a shop to get this fixed. The climate control system uses an inside and outside temp sensor to decide what kind of conditioned air you need. There is a main HVAC controller called a Remote Climate Control Module and the engine and body computers also play a role in the process. Then you have blend doors that are controlled electrically that open and close allowing air to pass thru the heater and ac boxes. All of these components and circuits may need to be tested to find the problem. And yes the dash could need to be removed to access some of those components. As long as the shop guarantees the repair will fix the problem, you should be safe. A dealer will have the most to lose if you blast them in public for not fixing it. A small garage, not so much.
Sounds like a flow flap or vacuum control head issue. The system uses 1 blower motor and one fan resistor. Both systems are intergrated to produce a BLENDED temperature. This means that to raise A/C temps originating at say 20 degrees to come out of the ducts at 45 degrees for cooling you need airflow through the heater core WHILE the A/C is on. This is done with a blender flap. Hot air that is potentially matching the water thermostat (165 degrees) is expelled to blend with the 20 degree air to raise the discharge temperature to 45 degrees and up. In winter, the A/C is engaged by design to keep the compressor lubed with oil. Usually this happens in Defroster mode, but through either Design or defect, may occur in heat position. Now some of the Digital Climate control units have a Diagnostic sequence you can activate with certain button pushing combinations. Research Your Model, Year and then Climate Control Diagnostics on the internet. The sequence is something Dealers know about. This Diagnostics would identify a malfunctioning Flap valve actuator. I recommend people to visit a Salvage yard which allows you to dismantle a car yourself and see how the car is built. Hope this helps and makes sense to you.
sounds like you have a bad temperature door actuator. This is a motor that connects to the door and uses inputs from the sensor in itself and the heater control module to open the door from hot to cold. I does take some knowledge on recalibrating this actuator and installing it. if it isn't install correctly it could damage the new part, so just take care
you say it will blow cold and hot air for only a few minutes. You don't say but I assume you mean will blow cold a few min. when turned on AC max , blow hot only a few min. when turned to heater hot. Assuming thats the situation and you say AC charged and shows good on the low side -you should charge the AC to a full charge on the low side will cause problems with the AC air compressor kicking on and off. I suggest while the engine is running and turned to AC cold max watch the AC air compressor clutch to see if it does indeed kick on and off if so charge completely. As far as the heater make certain you coolant is all the way to the top and the best way to do this is when engine is completely cold remove radiator cap and fill completely and completely close cap then fill overflow container just to the cold level. That should fix your problem.
I had one of these at my shop recently and I fixed it by replacing the heater control panel. I went for the resistor first with a known good one and it didn't take care of it. I bought a new heater control panel and that took care of the problem.