1996 FORD WINDSTAR CAUSE OF TROUBLE CODES P0171 & P0174

I have a 1996 Ford Windstar with a 3.8 litre engine. The OBD2 meter is showing trouble codes P0171 (System to Lean Bank 1) and P0174 (System to Lean Bank 2). This is after I replaced the gaskets (6 red rubber gaskets) between the Plenum and the Intake Manifold. I replaced the gaskets after having a diagnostics work up done at a local shop for $98 at which time I was told there was a vacuum leak between the Plenum and the Intake Manifold and that the gasket between the Plenum and Intake Manifold would have to be replaced. The mechanic called it the Upper Intake Manifold gasket. What else could cause these trouble codes (i.e. P0171 & P0174)? Thanks as any help would be much appreciated.

There is a revised gasket package that includes bolts etc! recheck that you still don't have a vaccum leak and if not then continue diagnosis for other causes of those codes on Fords!