Can a bad downstream o2 sensor be my issue?

My car has all the symptoms of a clogged cat, I know it's not my bottom cat, I have to check my manifold cat still but I was told that a bad downstream o2 sensor could make my cars computer act as if I had a clogged cat. I replaced the o2 sensor on my manifold that was $177 now the downstream one is only $85 But I don't wanna replace it if it isn't the issue, I was getting two codes before and now the light went off no code but car is still hesitint on acceleration and when driving up a hill or incline, the two codes were P0030 and P0420. I have a new manifold cat but I am hoping I can send it back if something else is the issue.