Bought a 2004 RX330 a few weeks ago with 145k miles on it. I've had a couple P0420 codes pop up, but only when I'm going up a certain incline (doing 25 mph - residential area) and I have to slow down to roughly 5-10 mph by letting off the gas. When I start to accelerate, it pops the code. I don't suddenly hit the gas, but gradually.

I hooked up my Carista OBD2 and ran OBD Fusion to see what was going on with the O2 sensors. My O2 sensor 1 voltage is 0, sensor 2 fluctuates with the throttle as I figure it should. Sounds like I need to replace sensor 1, correct?

 

Did you ever find a resolution to this? Exact same thing is happening to me.

I bought a 2006 RX330 with 270k miles for $600 with BOTH p0420 and p0430 (bank 1 and 2 cats) and found that the upstream sensors are reading 0.00v upon connecting my OBD2 scanner. So far I have replaced the spark plugs which were terribly gapped and cleared the code to see if that helps, but I'm just not sure what to do about the sensor reading 0.00v.