[XGamesJ6]

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