P0420 code, high B1S1 O2 voltage

Vehicle is throwing P0420, catalytic converter was replaced last year (but son drove it around campus for months with code returned). Live data from OBD reader shows B1S1 volts at 2.2 range, steady. B1S2 .6 range with occasional drop. catalytic converter temperature 500 area. Suspect B1S1 sensor but wondering about possible exhaust manifold leak. Ideas? Thanks.