0171 0174 please read

I have already replaced all necessary parts except valve cover. Is there anything else that can trigger lean left and right bank , also replaced o2 sensors,, ps did this work over a year ago please any and all suggestions thanks barry

What year is your van?

The MOST common cause of this code is a vaccum leak......which is any air that enters the intake that the computer does not know about.
There are so many possible causes of a vaccum leak......between the upper and lower intake manifold (loose bolts.....bad isolator bolts....etc), a leaking vaccum line (many of those), on newer (1999 and newer) leakage through the bushings for the IMRC shafts (requires lower intake manifold be replaced), defective PCV valve hose or breather hose (make sure that it is inserted into the flex hose).

Another possible cause that my '96 was prone to was dirty fuel injectors (a can of Berryman's B-12 Chemtool in a tank of gasoline to clean).

A weak fuel pump or other fuel delivery issue........change the fuel filter.

A dirty or defective Mass AirFlow (MAF) sensor, I have pictures posted in the pictures that my signature takes you to that show how to clean the MAF.
Also make sure that the Air filter box and the flex hose are closed....not leaking and that everything is snug.......