2001 Oldsmobile Aurora P0420 Check Engine Light Returning After Converter Replacement

My 2001 Oldsmobile Aurora V8 (79,000 miles, automatic transmission) recently developed a check engine light. I replaced the catalytic converter (universal brand), cleaned the fuel injectors, and the warning light initially disappeared. However, after two weeks of driving, especially on the highway, the check engine light returned. I'm now unsure whether the universal converter was low quality or if I should have installed an aftermarket unit instead. Additionally, I'm considering replacing the oxygen sensor—should I do that? How can I tell if my O2 sensor is faulty? I need help identifying which component is actually causing the P0420 code to reappear.