2003 GMC Yukon 5.3L Z Flex Fuel: P0171, P0174, P0420 - O2 Sensors or Catalytic Converter Issue?

I own a 2003 GMC Yukon 2WD with a 5.3L Z engine (flex fuel). The vehicle is currently displaying three diagnostic trouble codes: P0171 and P0174 (System Too Lean - Bank 1 and Bank 2) and P042-0 (Catalyst System Efficiency Below Threshold - Bank 1). I initially tested the Mass Air Flow (MAF) sensor using a method described in a GM-specific forum, but observed no significant changes. The MAF was also visually clean and showed no signs of contamination. I then performed a full cleaning of the MAF sensor, which did not resolve the issue. Following that, I replaced the fuel filter, after which the codes cleared for approximately two days and 200 miles of driving. Since then, I've thoroughly inspected all vacuum hoses and lines—no visible leaks were found. Given this history, I'm now wondering whether the problem could be related to the oxygen (O2) sensors or the catalytic converter. Any insights from experienced users on how to diagnose these components in a 5.3L Z engine setup would be greatly appreciated.