Crxsijoe

Hey guys, hoping to get some help. Got a P0420 code for my 2000 GSR. Catalyst system efficiency below threshold. So I replaced the catalytic converter. Still had to code so I replaced my upstream and downstream O2 sensors as well (NGK ones). Reset the Ecu and drove it about 25/30 miles. Went well. Then the next day after about 10 more miles got the same code. Any ideas what else it could be? Thanks for the help guys!