Cutting out the cat usually will only throw rear sensor code if it throws one at all. Often times it throws nothing but it will run richer. The front 02 is the one that controls air/fuel ratio and will be seeing more air and to make up for it will push slightly more fuel. Not a huge problem but one to expect. The rear simply monitors cat efficiency so it might throw a code indicating a failing cat but nothing to worry about. Whichever throws a code if any can usually be cleared by using spark plug non-foulers to space it out of the pipe a little bit.