P0420 Diagnostic and Fix for Toyota Land Cruiser - How to Resolve Check Engine Light and Poor Performance

Hi everyone, I'm facing a persistent issue with my Toyota Land Cruiser that's really driving me crazy. I've already replaced the front O2 sensor, but the check engine light remains on. Even after resetting the ECU, the warning comes back after just a few miles. Currently, my truck is experiencing significant hesitation and poor fuel efficiency. I've also recently replaced the spark plugs, cap, wires, and rotor—everything related to ignition has been checked and new. I don't want to replace parts blindly without knowing the root cause. While I'm comfortable doing mechanical work myself, I'd like to understand what's actually causing this P0420 code before spending more time or money. Any suggestions or troubleshooting steps for diagnosing a P0420 fault on a Toyota Land Cruiser? I appreciate any insights from experienced users or mechanics. Thanks in advance!