Anyone have experience with these codes? Mine have been on and off (mostly on) the past few months. Every few days they turn off but always come back on. Drives fine with them on but suspect it is in some type of limp mode when on. I know they suggest bad cats. I'm at about 225k miles. Cats seem quite pricey. The exhaust sounds ok to me. Inspection not due until May 2019. I used "Guaranteed To Pass" and it kept it off for inspection last May.

 

What year/make/model is your Lexus? I know on earlier versions (like my 2GS) this was a common problem. Under the heat shield on my y-pipe there was a pin hole that caused me to chase a P0430 code for months. You can google "lexus y-pipe leak" or something like that and it will give you lots of threads. However, I've only known this to be on early model LS and GS vehicles.

 

08 ls460 rwd

 

Most times these codes are false positive on the 400s so maybe it's the same on the 460. Usually there is a leak at the catalytic converter flange gasket. I used exhaust cement/ paste on my 400 and the codes never came back.

 

I had a P0420 code show up on my 2006 LS430 less than 2 months ago. Dash had CEL, TRAC OFF, and the Master Warning Light with Check VSC as a message. I found a leak under the clamp in the Y-pipe just past the O2 sensor. Took it to a muffler shop and had them cut out and weld in a new section of pipe on each side of the Y. Cost me $100 and my expectation is I'll never see the problem again. I am happy with the result.

Dash Lights

New Welded Sections

 

Same to the 'Y' pipe leak.
No codes.
I must have had the leak for a couple of months.