1. O2 sensors must be at a certain distance and with spec resistance to function correctly
2. If the secondary O2 is throwing a code it can be resolved
3. If it is the primary, replace it after comparing each wires resistance against spec and determining if it is bad.
4. Which O2 did they weld, that part is not to clear, and more specifically where did they weld it?
5. It is usual to throw the secondary O2 sensor code after replacing the cat, but can be bypassed.