I did this write up on the 2014 trucks thread but it's more fitting here. Mods if you want to delete it over there please do. This was for my 2014 Sierra Denali 6.2 ARH long tubes, Y pipe with cats.
Yeah I know it's stupid to not tune this mod but I'm doing it for possible warantee issues. With it being a brand new motor design I don't want to foot a bill GM would normally take care of. They can't void a warantee if you don't touch the tune. Here is what I did to make the CEL go away.
There are a lot of threads on the interwebs that address the P0430 and P0420 inefficiency dtc codes. A lot of guys use plug defoulers so I started experimenting with them to no avail. I found one thread on GTO site so I sorta copied the idea and so far it worked. I'm 50 miles and 3 heat cycles in and no CEL.

Parts needed:
4. Auto zone 18mm spark plug defoulers
2 Wix 33050 fuel filters.
Small washers

I'm about 15$ in parts total

I bought the short and long length defoulers to experiment. Drill out the short defouler with 1/2 drill bit and use a Dremel to make the 02 sensor fit down inside.
Then take the fuel filter and remove the outer portion so that you only have the cone and base of it.


Place it in the longer undrilled defouler and add washers to secure it. Stack the drilled defouler with the 02 sensor installed and install it in the down stream 02 bung.



I have catted long tubes so if you don't have cats then you may need to use two long defoulers to pull it further from the exhaust flow.

I'll keep everyone updated if I throw codes

 

Interesting... where did you come up with this idea?

 

500miles in no CEL