What is the reason why the car air conditioner cannot switch to internal circulation mode?

[BrodyFitz]

You need to turn on the air conditioner first before activating the internal circulation mode. Here are detailed explanations about the car's internal circulation: 1. Definition: Internal circulation is an operating state of the vehicle's air conditioning system. In this mode, the air exchange channel between the interior and exterior is closed. When the blower is off, the air inside the cabin doesn't circulate; when the blower is on, the intake air comes solely from the cabin, creating internal air circulation. 2. Principle: Vehicle air conditioning systems typically have an electric or manual internal/external circulation switch. In external circulation mode, the AC draws air from outside the vehicle, while internal circulation reuses the cabin air. If the circulation switch uses a mechanical lever instead of a button, you can position the lever midway between internal and external modes to allow moderate outdoor air intake, which improves air quality while enhancing AC performance.