Ford Taurus
Heater Cores and Blower Fans
Ford Explorer XLT
Car Heaters

What would cause the AC and heat to only work when switched to the high position?



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You have identified the problem when you state that it only works in the high position. Your switch has variable speeds, it contains resistors that control the speed of the fan, for both AC/Heat. These resistors are burned out/or the connector to the resistors has become unplugged. The resistors are located in different locations so please get your auto manual to locate there position, and verify there operation.
Most cars have a simple blower motor resistor that provide the low-medium speeds. If it burns out the fan defaults to high speed - the most safe 'default'.

Walk into any auto parts and tell them your car only blows high and they'll immediately know what you need.

In most Ford vehicles this is a 10 minute, 2 screws procedure that most anyone can handle.