P0420 error code on GS 300 MK3 2005

[AkkiDonz]

Hello mates,
First of all i agree there are so many post regarding P0420 and i have tried most of the suggested tricks. I am at the end stage to change my Cat. Before doing that decided to check here for the last opinion. So i only get P0420 code on low boost (city driving). If i reset the code and drive motorways no code. 
I have changed spark plugs, welded the pipe(funny they found leak on bank 2 after o2 sensor), Cataclean, o2 sensor has been replaced by old owner few months back.

Any other suggestion ?

Thanks 

[ColinBarber]

In my experience I've found the following things produce a P0420/P0430 error:

Hole in the exhaust

Upstream O2 sensors (not the downstream sensor that is reporting the issue) or potentially an air leak in the intake system/MAF sensor 

Cat converter.

 

The error doesn't always come up because the ECU only checks for cat efficiency during certain driving situations, and if you have a cat that has lost some efficiency then it can be borderline if it passes the test or not.

[AkkiDonz]

Thanks Colin,

I have checked for exhaust leak and they found no leak but some time with open window I can smell nasty but very rare.

will check with O2 sensor.

[AkkiDonz]

@ColinBarber
Now i am getting bunch of codes. P0138,P0158 and P0420. 
Before if i reset the codes, it wont show up that quick. Now as soon as i resets it start appearing again. 

[ColinBarber]

P0138 normally means an electrical short, either in the wiring or the sensor itself. It's a hard fault which is almost instantly detected, which is why it comes back straightaway. That and the other codes point towards a damaged downstream sensor.