[3.1L Z]

My Foxwell scanner describes it as "Mixture control exhaust gas after catalytic converter too lean. It only occurs on long drives...say 3 hours or more. Obviously 2 sensors up front on the manifold..Is there just one in the rear? Also, am I correct in assuming its the rear that is faulty? Car runs fine and CEL goes away when i put "cataclean" gas treatment in it..

Which O2 sensor should I replace?
No other codes..Its an N52 btw

Thanks