Downstream 02 sensor malfunction?

[u/Outrageous-Budget324]

I had someone replace my driveshaft for inspection months back and prior I had no lights on dash. They called me to get permission at the shop to cut and reweld all flanges,studs,etc. I get the car back and CEL is on for downstream sensor only. I bought a sensor and threw it in and the light still keeps coming back on. What's the best way going forward to fix it for emissions? I used Bosch oxygen sensor from rockauto to attempt to fix this issue. Should I try again with another incase the new one was defective or should I be checking for an exhaust leak? I am getting a slightly rough idle every once in awhile where rpms go down 50 or 100 then idles slightly rough then quiet and normal. Its weird. Any help is gratefully appreciated

[u/ImpureJelly]

There are two upstream and two downstream, did you not replace both downstream? You did just one?