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When my car is shut off, I hear a thump sound at the dashboard. I later learned this is the sound of the vent flaps going into the closed position. Why do the vents automatically close when the engine is off?

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  • Welcome to Motor Vehicle Maintenance & Repair! Feb 11, 2022 at 10:47
  • If you opened your car and found mice in there, you would be asking why the vent flaps don’t shut when the car is shut off.
    – HandyHowie
    Feb 11, 2022 at 13:53

2 Answers 2

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Most vehicles which you hear the vents close after shutdown is due to the vents being vacuum operated. They are vacuum operated in one direction and have a spring which returns them to a "home" position. Once the vehicle is off and the vacuum source is lost, the flaps automatically return to that home position and that's the noise you are hearing.

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  • Many newer vehicles use electric vs. vacuum operated motors to move HVAC dampers. But your point is still valid, many of these are programmed to return to "home" at power down. I've noticed that some vehicles do that and others don't.
    – jwh20
    Feb 11, 2022 at 15:27
  • @jwh20 - Good add for the electric versions. I've never experienced what you're talking about, however, after you mention it, I can see where manufacturers would do that. Feb 11, 2022 at 15:43
  • The home position is usually in the defrost position commonly called the default position. If an electric version detects a fault it will automatically try and go to the defrost position until the fault is repaired. This is a safety mechanism so because defrosters are important for visibility.
    – Jupiter
    Feb 11, 2022 at 17:07
  • If the vaccum system is in good shape, vacuum will be retained after engine shut-down for seconds or minutes. Hearing the vent motors return to their home positions immediately suggests there's a leak in the vacuum system. Feb 11, 2022 at 21:34
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I know that older General Motors vehicles (2006 Impala) did not have this function. It has been added to newer models at some point (2018 Silverado). An issue develops as the older HVAC door servo motors age. If power is removed (battery power) while the doors are not in the home position, they get stuck while trying to home after power is reapplied. In some instances, they can be rehomed in others they self-destruct shearing the internal gears. Allowing the software to home the doors after the ignition key is removed may prevent this.

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