정비 사례P0420

I am trying to troubleshoot a 4runner with 135k miles, auto, 3.4 V6 California emissions. This is the freeze data: PID Description Value Units 0x02 Freeze frame DTC P0420 - 0x03 Fuel system status Closed-loop, using oxygen sensor feedback to determine fuel mix - 0x04 Calculated load value 34.12 % 0x05 Engine coolant temperature 197.6 F 0x06 Short term fuel % trim - Bank 1 0.78 % 0x07 Long term fuel % trim - Bank 1 3.91 % 0x0C Engine RPM 2287 RPM 0x0D Vehicle speed 68.97 MPH 0x0F Intake air temperature 95 F So far the front and back Ox sensor was replaced and the MAF cleaned to clear a P0171 co

My 2005 Buick Century with 353k miles gets a recurring P0420 code, catalytic converter operating below efficiency level. Twice this year, I tried using Cataclean, but with no success. I noticed a small hole in my muffler at the seam. Could a lack of exhaust back pressure cause the CC to operate inefficiently or improperly? Can anyone recommend a catalytic converter cleaner? Or, is it all snake oil?