206 P0030 Error Code

I received the P0030 error warning on the 2006 model 1,4i vehicle. Although I replaced the oxygen lower sensor with a new one after the warning, the problem was not resolved. We connect the fault to the computer and delete it, and after giving gas for a while, the same error code reappears. If there are friends who are met with this situation, I would like to receive information. What could be the problem?

I replaced the catalytic converter with a new one, as I could not pass the exhaust inspection . We noticed that the lower sensor became unavailable during the converter replacement. So both sensor and converter changed, the values returned to normal, I was able to pass the examination. I think there might be a different problem. I'm going to have the power line and the vehicle sensor socket checked, I don't think of anything different.