I need some expert advice, I am at a loss. First some basic details.
My Car:

2005 G35 coupe 6mt

Modifications
Z Tube
K&N Drop-In Filter
Motordyne 5/16 Upper Plenum Spacer
Copper ISO-Thermal Throttle Body Spacer & Engine Gaskets
Strup Polished Stainless Steel Headers
Fast Intentions Stainless 300 Cell Metallic Cats
Fast Intentions Stainless Intimidator True Dual Exhaust
Technosquare ECU Reflash

I have had all of these parts on my car for over 2 years including the technosquare tune. I started getting P0420 codes shortly followed by P0430 codes. I assumed I need to add defoulers to my cats so I did this. Then I would get consistent P1273 codes.

With defoulers off: P0420 & P0430
With defoulers on: P1273


I have checked for exhaust, cats, headers leaks and did my best to search for vacuum leaks. I have removed the cats and they are in good shape. I even brought the car to the dealership and they want me to put my stock cats back on the car. I replaced the B1S1 02 sensor.

Any info would help greatly. If anyone knows of a shop on Long Island, NY that deals with G35’s I would appreciate it.

Thanks