Bank 1 is the upper oxygen sensor(s) before the catalyst (catalytic converter) bank 2 is the oxygen sensor after the catalyst (catalytic converter)
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.
Trouble code P0171 means:System too lean (Bank 1)
Bank 2 running too lean.
po171--- system to lean (bank # 1)
Trouble code P0171 means:System too lean (Bank 1)
P0171 System too Lean (Bank 1)
Trouble code P0174 means: System too lean (Bank 2)
Trouble code P0171 means:System too lean (Bank 1)
Trouble code P0171 means:System too lean (bank 1)
Trouble code P0174 means: System too lean (Bank 2)
Trouble code P0174 means: Fuel System Too Lean (Cylinder Bank 2)
Trouble code P0171 means: System too lean (bank 1)