S10 rough engine

Cylinders 1, 3 and 5 are working correct. Looking on the spark plugs shows
that cylinders 2, 4 and 6 are working too rich (black dry sooty). But the
engine code is P0174 (lean bank 2). I have tried to find an exhaust leak from
the manifold to the O2S sensor without finding anything. The engine is going
very rough and the power is less than usual.
I have changed the O2S sensor to a new one. The same result.
I am only looking on the passenger side of the engine. Correct?
What more to look for which can give this combination of too rich spark
plugs  and a too lean code?