P0420 - Engine code and engine management light on

[Ger88]

Hello

Was looking for a bit of advice on what action to take for an engine code error: P0420 - engine light is on permanent but car seems to run fine - I have a Vgate pro OBD2 interface with the OBDll auto doctor app.  Is this the best app to use and what would I be looking for in the live data readings etc?  With a paid app I assume I can rule out certain issues and/or slim it down to, or maybe even pin point the exact problem?

2005 xtype 3l 

Thank you  --  Ger

[Kie]

That code is for a catalytic converter running below maximum efficiency which can sometimes be caused by a faulty oxygen sensor or worst case the cat has had it. Your O2 sensor voltage activity live data (upstream and downstream) should show if it's the cat and whether it's bank one or bank two but test the sensors first.

Regards, Kie

[Ger88]

Thank you

Would you happen to know which tool is best for this, or app - I was thinking OBD auto doctor, or torque?  It's saying bank 1 so that's a start but could be anywhere along that line - surely if a sensor had gone I would have a code for that?  Or do I run a test on them?

Kind regards