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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
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.