Cel code
 

What is the fix for the P0420 code? Two days out of the shop, $5,200 repair bill (paid $5,000 for the vehicle) later and I get a CEL!

Take it back to the shop.
A $5200 repair bill and it throws a code shortly after leaving the shop. It appears to be related to the catalytic converters

What work did you have done and was this code present before the work was done?

P0420 is a low conversion on the three way cat on bank 1. While this is an expensive problem (cats are retailing at over $2K as parts), it may or may not be related to the work you just had done.

I keep getting a P0420 code on my 996. It started after replacing the OEM exhaust with a Fabspeed system (cats, mufflers, headers, X-pipe, etc.). Have tried everything. It comes back after awhile after I clear it (sometimes a week later, sometimes a month later). Try putting on an O2 sensor extender. That seems to work for me.

What is an "O2 sensor extender? Which of the sensors is it (location)?

It is entirely possible that the cat code is unrelated to any work performed. Sucks, but possible.

Find low mileage cats. Way cheaper.