rishilohi

It is very hard to troubleshoot P0430 and P0420 , when it came in my lexus ls400 1999, i tried replacing almost most of the fuel system parts and yet did not find any solution.

Recently i replaced the PCV valve and the air filter, It does make difference in the performance.
Yesterday i found out that my CAT gasket has a leak and when i replaced the gasket the vehicle was absolutely quiet. I deleted the code and after 160 Miles also the code did not come.

I recommend all of you to kindly fix the small small issues then only look at the CAT.
1- Air intake leaks
2- dirty air filter
3-PCV valve
4-gasket leaks
5-silencer leaks
6-oxygen sensor ( only after checking the readings )
7- CAT ( confirm after checking all of the above )

billydpowe

iTrader: (1)

your cats, and o2's will probably do the trick

Banshee365

Do you mean his cats and current o2's are fine? If so I agree. If you're saying replacing the cats and o2's will do the trick I sort of disagree.

The cats in these cars are very robust and usually only fail after serious neglect by running under a misfire for a long time or something like that. MOST 420/430 codes are caused by leaks at the flanges. Some, like mine, are not. I've still yet to spend the time to figure out exactly what mine is doing as it runs and drives fine and I'm still recovering from all the time I put into the building the stupid thing.

billydpowe

iTrader: (1)

yes his are ok , as I think about his 99, I think I would replace the temp sensor, not costly and can make a real difference...

dicer

Bad cat converters cause it too.

hardy2

Try installing non foulers on the o2 sensor should do the trick.