The most common issue with heat or A/C in the late model Ford windstar is a broken blend door. A broken blend door actuator is almost as common as a broken blend door itself. Typically, the fix requires disassembling the entire dash and replacing the whole plenum box, which can be a costly repair. Linked here is a video from www.heatertreater.net demonstrating how to access components of the plenum box for repair without disassembling the entire dash.
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Take it apart to determine the cause. Only then you will be able to fix it.
The heater on the 03 Mazda Tribute is going in and out. What is the problem and how can it be fix?
heater windstar ford
The "Related Question " below has much discussion, including a temporary fix, for the coolant bypass hose.
The only fix for a leaking heater core is to replace it.
Heater circuit fault
Fix wind star climate control switch
The only fix for a leaking heater core is to replace it.
15.0 liters ( 15.9 quarts ) according to the 2001 Ford Windstar Owner Guide - cooling system capacity with rear heater
Fix wind star climate control switch
If you are getting a Lean code then it is probably being caused by the intake plennum gaskets. This is a common problem on the Windstar. The fix is relatively easy.