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2013 F-150 XLT 4WD with 5L
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Air distribution in your truck is determined by the function selector switch position (manual systems), or is controlled by the EATC system in the AUTO mode (EATC systems) which can, of course,be overridden by the driver if desired. Airflow control doors are used to direct airflow within the air plenum chamber. Vacuum control motors are used to position these airflow control doors.

The air distribution system is designed to provide airflow from the defrost nozzle when no vacuum is applied to any of the vacuum control motors. This is done to prevent a situation where defrost cannot be obtained due to a system vacuum leak. With that in mind, check your lines for one with a hole or is disconnected.

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