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Q: What could be causing to heat to only come out of the defrost vent?

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When the heat is on the air only comes from the defrost vent and not the front or feet vents.

My car has 56000 miles.
My car has an automatic transmission.

Air is directed to the floor, dash vents, and defroster by vacuum controlled blend doors behind the dash. When you select where you want air flow on the heater control head, vacuum is directed to different vacuum connections to move the blend doors one way or another to redirect airflow. There should be a vacuum supply line from the engine to the heater box under the dashboard. It would most likely be located on the passenger side of the firewall under the hood. Check to make sure it is connected and/or not broken. If the vacuum supply is good, then the lower dash panels on the passenger side would have to be removed and the vacuum lines and connections are good. It can also be possible that one or more blend doors in the heater box can be broken or binding, as well as the heater control unit itself may be bad. If nothing obvious can be found, have a certified technician look into the vent airflow issue to find the exact cause.

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