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Super Moderator
1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
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16,974 Posts
What year and model? Generally this is from a failing vacuum check valve that is near the intake manifold or brake booster and goes into the dash. When manifold vacuum drops under load, the valve keeps good vacuum in the HVAC control lines for holding the selected mode. It will always default to defrost for safety if the vacuum drops.
It should be pictured on your underhood vacuum map.
 
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