rapidom

Hi everyone, from december 2020 i have a fault - 6093 - P0420 00 [237] - catalyst system efficiency below threshold. What i have done to fix this problem? Check out and PLEASE HELP ME - reseted ECU values then i drove on PB for 250kms and fault was again active. - i have changed spark plugs - changed both O2 sensors to used OE with low mileage - new catalytic converter (not OE) with EURO 5 emission - checked plugins to ECU - and the last step was new air filter and oil separator (because one day it caused and oil leak. What else i can check? My mechanic doesnt know what else he can do... i asked him to check exhaust after catalytic conv but he told me it doesnt matter because its after 2nd O2 sensor... Also he told me that he cannot see anything wrong, air leak or something ...