Troubleshoot ac returning to default

the ERG shouldn't have anything to do with what vents the AC comes out of, rather the dash switch that exists for changing from defrost to main vents to floor vents controls what vents are used.

If the air flow cannot be controlled from vent/defrost/floor you may have a blend door issue. This is an electric motor that operates a door in the heating system to control airflow. There are usually 3 motors controlling airflow direction,heat and recirculation. The Ac not blowing cold may also be another problem. If the AC is operational, the compressor will be engaged and the Ac low pressure line under the hood will bw cold. If this is all ok then a blend door issue should be suspected

Check to make sure the engine vacuum is getting to the heater/ac control. Bad vacuum hose or leak. Many vehicles use engine vacuum to move heater doors and controls. When the vacuum is absent, it usually defaults to air blowing out the AC vents.

The flow switch it change's howthe air flows get a motor manual at auto parts stir about 25 bucs it will tell u what up

control panel or mode door actuator is bad, when bad system defaults to defrost floor,

I think your problem is the head unit that selects where the air comes out ie. Face, floor, defrost, or hi-level face and floor. If that's not working then it will not let you switch location that you want the air to come out.

go by liquid coolant and check ur heat and ac fan it might be stuck or broken

Greetings, I read your question and I think Reliable-Store can help you in this. Here's the site link.
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when your car is in defrost it has an ac cycling it removes the moisture then sends the air over the heater core depending on how warm you set it you can feel warm or cold but the ac is always cycling...ridding the moisture...you need a good ac tech it will be okay...its just expensive going into the heater core if needed...!

Inspect all of the vacuum lines in the AC system under the hood. If one of these lines is cracked or broken the AC system will not be able to switch the vents over.