2003 Ford Taurus P1131

Had the P1131 code scanned at AutoZone, with the definition lack of bank 1 o2 transition and explanation ECM detected a bank 1 o2 at its lean limit with probably causes [1] misfire bank1 [2] vacuum leak Bank1only [3] Fuel Injector problem bank 1 [4] engine mechanical condition.

Querstions- which is bank 1?

Already performed actions - repaired a vacuum leak on rubber connector where hose from PCV valve enters intake manifold. Replaced O2 sensor on the back exhaust manifold, the one near the firewall.

What to do next?