Howdy, I am helping a buddy with his 13' G37, and he has a P0420. I was thinking faulty o2 sensor, so I looked up the voltages and they line up with being fine, but the cat being bad. Can we get rid of the light with an o2 spacer, or is there any other cheaper way without replacing the cat

o2 Voltages when hot

o2 Voltages when cold

 

Here is a video of showing a good voltage graph of the rear O2 sensors from a G37:

Use this to compare it to your buddies.

Keep in mind, on the G37 the "sensor 1" is a wide band / air/fuel sensor and will output 2.2 volts with an AFR of 14.7 / lambda 1.0
The sensor 2s are the old school "o2" narrow band sensor and will output a voltage of almost 1.0 when rich, and almost 0v when lean.