04 altima with p0420 helppppp

[q45bowlinggreen]

Bought car with bad motor has 168k i bought diff motor complete tore itapart rebuilt it new bearing piston rings valve guide whole 9 yards put itcar runs great pulls strong drove car about 100 miles today after installing motor ses came on code is p0420 so i took convertert off and o2 sensor after precat of my buddies 06 altima that runs drives with no codes put on mine restet ecu drive about20 miles same code comes on ??????? Maybe ecu neeed reprogrammed