P0420 Catalyst System Efficiency Below Threshold (Bank 1)

Hello, My friends vehicle is throwing a P0420 code. Catalyst System Efficiency Below Threshold (Bank 1). I'm fairly certain that it may just have a bad o2 sensor. The car gets bad mileage but the idle is pretty steady and smooth. They say it definetly feels under-powered.

I'm almost completely sure that replacing the o2 sensor would all these issues. What do you all think?

Also I just want to make sure is the sensor BEFORE the cat on the same side as cylinder 1?

2004 Chevy Tahoe 4WD ZR1
I'm pretty sure with the 5.3L V8

Hello, My friends vehicle is throwing a P0420 code. Catalyst System Efficiency Below Threshold (Bank 1). I'm fairly certain that it may just have a bad o2 sensor. The car gets bad mileage but the idle is pretty steady and smooth. They say it definetly feels under-powered.

I'm almost completely sure that replacing the o2 sensor would all these issues. What do you all think?

Also I just want to make sure is the sensor BEFORE the cat on the same side as cylinder 1?

2004 Chevy Tahoe 4WD ZR1
I'm pretty sure with the 5.3L V8

with this problem and this cat converter code remove the upstream O2 sensors both ...mark which one is driverside/passengerside...with the holes open go for test drive see if the power returns then reinstall...then remove the downstream O2 and swap locations to see if the code changes...if the code does not change the O2 sensors are good. that would mean the cat converter is causing the code...
some have had converters break up and then plug up the muffler !

checking for this when the converter is replaced is a good idea..