Rough idle help

[Crocodile]

I have a p0420 code and rough idle. Seems to be way tougher when A/C is off and when it’s on it dulls the idle and shutter a lot. What could be the cause of the rough idle?

 

[gossie]

Simple things are plugs and dirty air cleaner.

 

[Martbar]

As above google liked a leaking exhaust manifold/s as well.
Bank 1 right hand side facing front of engine.

 

[Fu Manchu]

https://forums.justcommodores.com.au/threads/p0420-rough-idle.300301/

 

[Fu Manchu]

There are other threads on this and we have been trying to help you.

Eg. https://forums.justcommodores.com.au/threads/engine-light-100-of-the-time.300194/page-2

You need to explore the suggestions already posted in the other threads or else you’ll be posting threads on it for years to come.

 

[lmoengnr]

Lots of possibilities for this one...
Easiest one to check out would be an exhaust leak from the gasket between the L/H cat and exhaust midsection.