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This is a default setting when the AC system no longer gets vacuum from the engine. You have a vacuum leak from he engine to the AC controls. The mode door that changes the vents the air comes out of is vacuum controlled. Find the vacuum line on the passenger side of the engine near the fire wall and evaporator case. There will also be a gray vacuum line. Follow it from the engine to the fire wall under the evaporator case. The look under the glove box at the fire wall and follow the vacuum line to the AC control panel. Most of the leaks we find are from bad/broken vacuum lines under the hood around the engine.