1997 Hyundai with O2 sensor 2 codes

I have a 1997 hyundai elantra with 138K miles. I bought it recently with P0136 and P0141 codes and other issues that need to be fixed before passing inspection. The front exhaust was leaking in the flex pipe area. I installed a connector as a temporary repair until I can determine whether inspection is worth it. The exhaust sounds OK, but I'm not sure it is 100% sealed. Do the codes mean anything if there is any exhaust leaking?

After "repairing" the exhaust leak, I waited for warm weather to address the O2 sensor code issues. Some 2K miles later, the P0136 code (O2 sensor 2 ciruit malfunction) was replaced by P0137 (O2 sensor 2 circuit low volts). Another 2K miles later, I now have only a P0138 code (O2 sensor 2 circuit high volts) and the P0141 code which has always been there. Assuming these O2 sensor 2 codes are meaningful with a possible exhaust leak, is there anything to do before replacing the O2 sensor 2 and/or exhaust pipe? I don't want to spend $500 to repair a $750 car that still might not pass inspection because the cat converter is shot.

1) Have the exhaust leaks fixed and welded at a local muffler shop.
2) Check the fuse for your H20 sensors.

If the catalytic converter is shot isn't it throwing a DTC?