mruck -
Hmmmm - You had the 171/174 codes on a 2000 Windstar, and they went away with only a fuel filter and spark plugs?
Quick prediction: The codes will be back soon, unless you've also taken care of the infamous "leaking isolator bolt" TSB. My 2000 needed this update, at about the 60K mile point.
Required reading: While this procedure is designed for the do-it-yourselfer, it will also help you make sure the shop solves the root of this problem:
http://leckemby.net/windstar/windstar01.html