My 2002 had a P0430, and there were two causes: 1) a small gasket leak at the front of the Y-pipe, 2) a small crack in the passenger side of the Y-pipe, directly underneath the very front of the heat shield.
Every LS430 over 10 years old and driven on salted roads will develop this problem. The heat shields, heat shield clamps, and exhaust pipe are made of different metals, which encourages galvanic corrosion.
The mild steel heat shield clamps are in close contact with the stainless steel heat shields. This + salt quickly rusts off the clamps and cause them to rattle against the pipe. Because the stainless heat shields are in close contact with the mild steel pipe, I believe this also encourages corrosion of the exhaust pipe. With the clamp loosened, it is even easier for salt water to get inside the stainless heat shields, which burns off, and then the remaining salt encourages rusting from the inside.
See thread at http://us.lexusownersclub.com/forums...0420-on-ls430/
If you catch the problem too late, it will cost you $300-600 to buy and install a new Y-pipe. Cheaper to catch the problem early when the muffler shop can still weld the crack. If the crack is too deep/big, it is usually not possible to weld. You can get a brand new pipe+catalyst for the essentially the same price as having a shop cut off the bad pipe, fabricate a replacement, and weld the replacement in.
Also, don't bother reattaching the heat shields unless you do a lot of offroading and driving over dry grass.