If you have the temp wheel set in the "white" area, the CCU can shut down the center vent and redirect the bulk of the air to the side vents and "leak" to the defrost nozzle when the fan is ramped down and the heat is cycling. What it does and how it does it depends on the temperature inside the cabin and the ambient temperature outside, sensed in the intake plenum near the firewall. The same setpoint on the temp wheel will behave differently on a cool day and a hot day, despite maintaining the same temperature inside the cabin.

To test your system, stick the temp wheel firmly in the blue, say around 18˚ and see how it does. It should keep the center vent on all the time and the fan shouldn't drop to the lowest speed unless it's getting pretty cold in the cabin. When in the "blue" area, the defrost nozzle should not be open and the center vent should always be open.

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