My Subaru does not have auto heat, it is manual. When I turn on the heat it works fine on the drivers side and blows cold air on the passenger side.
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Is this a dual zone system by design (with different temperatures set between driver and passenger?). Do you have a photo of control panel? – zipzit Jan 17 '20 at 17:09
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As there's heat on one side only, the bits in common (the heat exchanger and its plumbing) are working correctly. Something in the mechanism that controls the side-to-side distribution is broken. This could be either the side-to-side blend door itself, or whatever controls the door position; the vacuum lines/motor/control switch, or electrical lines/motor/switch, or physical control knob/lever and control cable. – DavidSupportsMonica Jan 17 '20 at 21:53
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Welcome, there's mix doors and blend doors behind the dash and more of them on the dual zone climate control. As mentioned above a photo of the controls would be good. – ajayel Jan 17 '20 at 22:46