Catalytic converter

I have a 2006 hummer h3 with about 139000 miles it hasnt been giving me any problems for months but check engine light wont turn off after I reset it, I was told by dealer that I need catalytic converter replaced ive replaced yhe sensor on manifold cat, the one that you reach from the top through engine there is total of three, one more below on manifold cat and one after other cat under car between cat and muffler, they gave me same scan code autozone gave me (p0420 low effic bank 1) should I replace other sensor or rplace bottom cata conv