What do i look for with a code reading catalyst sysytem efficiency below threshold bank 2 in 2000 Nissan maxima?

If that is the only code you are getting and the car has high mileage (90k +), the odds are in favor or the front pre-catalyst needing replacement. That car has 2 pre-catalyst converters, one for each bank, that run from the exhaust manifold towards the catalyst itself. The first one (also called front bank, left bank or bank 2) is the closest to the radiator. The second one (also called rear bank, right bank or bank 1) is the one right before the catalyst. That code is for the front pre-cat wich has a limited life of service before getting clogged up and needing replacement.