I have no cat on my 1997 DX civic and I do not have a code for the secondary O2.
borrow a known good O2 sensor, or buy a cheap one off ebay or rockauto.
It did pop up on a buddy's 96 or 98 civic ex one time, we fixed it by getting a 90degree O2 adapter for $15 off ebay. Seems the EX's are a bit more sensitive than the DX ones that have a cat for a manifold in factory form.
If you want to avoid random crap, grab an aftermarket magnaflow catalytic converter, as a stock one is old enough to be worthless anyways.
You will not notice a power difference in either case except during very intensive all motor builds.