Replace Catalytic converter and P0420 still there

These newer cars are super sensitive about the performance of the cats, and O2 sensors. Did you use OEM Toyota cats and sensors, or aftermarket ones?

If you didn’t use the Toyota OEM parts, then it’s usually due to the replacements not playing nice with the car. You really need OEM sensors and cats on the newer cars, sadly. It was fine 10+ years ago when the computers weren’t as touchy, but not after the EPA (and governments around the world) got picky about how forgiving some of these ECUs were. For really old cars (think 90s-early 2000s) it isn't an issue but these new cars will never work right with aftermarket sensors*.

*If you’re in a no-inspection state, just accept the P0420 code if an OEM sensor does not help and the cats are bad ;). We do it a lot in these states anyway after 10+ years because of how expensive it is to TS the system. It's normal. Unethical? Probably. Illegal? Heh, nobody pays attention.

This car is one of those new-fangled whiny ones, so OEM is a must. If you're lucky, it may be early enough to not be overly sensitive but unlikely. That said I suspect it is sensitive because it is a Lexus and Toyota is known to be fickle about those things on the Lexus side.