Q: Changed catalytic converter, no lights on dashboard, but codes for catalytic converter & O2 sensor still remain

Hi there, thanks for writing in. I am not sure why you changed your catalytic converter. Assuming you had a P0420 code, replacing the catalytic converter should of resolved that code. The codes originally stored need to be cleared. The fact your check engine light remains off tells me, you may have fixed the problem. Try disconnecting the negative battery terminal for at least 2 minutes, reconnect. Start and drive the vehicle for 10-15 miles. Re-check for codes. If a code returns on the first drive cycle (pending code), a problem still exists. The check engine light should come back on (the next drive cycle) if, the fault is recognized by the computer. An exhaust leak before the oxygen sensor will cause O2 sensor and catalytic converter codes. If you are having trouble identifying the fault, consider having an expert automotive technician from YourMechanic come to your home or office to inspect and diagnose this issue for you, and make or suggest any repairs as needed.