2003 altima se sensor issues P0420

[altima03beast]

Hey guys , i have a 2003 altima se 3.5 my mods so far are nismo cai and OBX headers downpipe NO CATS. I got the Headers about a month ago because my cat went i also believe that the 02 sensor bank 1 went as well and i keep getting that p0420 code. i recently had an 02 simulator installed to correct the problem and the light came back on. i hate when this code comes back cause there's alot of lost power when this happens . do you guys think i should replace both sensors cause i know that 02 sensors are still there so im thinking they're bad. so what you think i should do and let me know if anyone had this problem