I replaced my intake manifold gasket and catalytic converter on my 2002 Chevy Impala and need help.?

When putting things back together, I ended up with one extra connector that is dangling (there is a set of 3 wires that lead to the alternator, 2 of which connect to the alternator, and there is no spot in the area within reach of the wiring for the connector to attach to), and I cannot figure out where it is supposed to go. My check engine light stays on, and the code reads that the catalytic converter is defective because the oxygen sensor in the front is reading different than the oxygen sensor in the back. I am just learning how to do car repairs, so please don't judge me harshly, but can someone please help me figure out what I need to do? Thanks!