Hi, diagnostic trouble codes almost never point to a specific component and indicate it has failed. They do, however, lead down a path of diagnostic testing related to that component. In the case of "oxygen sensor codes", it's quite possible the circuit related to one or more sensors (a V6 engine equipped vehicle has 4) is showing a failure, or that the related fuel trim readings that the sensors work to measure are showing out of specification. This would lead to testing. A poor reading, for example, could be due to a small exhaust leak present after the replacement of the catalytic converter. You can change sensors all day long, but if there is a leak - you'll never get correct readings.
Additionally, what is the quality of the sensors used and how have they been wired? If you are not using genuine GM parts - you could be asking for difficulty, as aftermarket oxygen sensors in my experience tend to not work as well. You'll want to have access to a diagnostic tool that can monitor and measure performance of the components that are said to be failing - so you can determine what steps must be taken to resolve the problem.
An experienced technician at a local shop near you could be quite helpful to you.
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