My 120k 2004 ls430 pops the 0430 catalyst efficiency code. Graphing the 4 oxygen sensors SEEMS to show the downstream bank 1 sensor at a pretty consistent 0.8V, it the downstream bank 2 at only 0.06V. It doesn’t oscillate with the upstream sensors, which I’m led to believe is an indicator that the cat is NOT bad. This is all at a warmed up idle. The bank 2 sensor 2 will shoot up to around 0.8V during driving etc, so I believe it works. However, it will always eventually return to around 0.06V. Car came with paperwork indicating work on the y-pipe previously. Does this seem like an exhaust leak, which is fooling the banks 2 sensor 2? Even last dealership visit noted the p0430, but recommended replacing an 02 sensor, no mention that the cat itself should be replaced. I did replace upstream bank 1 for a heater control code, and it has been fine since.

Warm idle graph

Warm drive graph

 

I think you might be right with the possibility of an exhaust leak somewhere. Certainly worth checking thoroughly for leaks to rule it out.

 

Yup, exhaust leak, normally in the join between the CAT and the Y-pipe or else a crack/split in the Y-pipe between the join and the downstream O2 sensor.
Air is getting into exhaust between the CAT and the downstream sensor.

I had P0420 on mine and it was a crack in the weld joining the flange of the Y-pipe to the Y-pipe.
They tried to re-weld it, but ended up making a new Y-pipe section for me.