2001 motero sport 3.0 check engine light
 

my moms 2001 mitsubishi montero sport is getting a PO421 code i beleave its the pre-converter catalytic ( the Y looking thing lol) i had the O2 sensors tested and they open and close as they should. does the whole converter need to be replaced or only the side that is bad in my case its bank 1 from what im told is the right side? does anyone have a part number?