Threw a PC420 and PC430 CEL's...
 

Running 5/16" MD spacer, UR HFC's, and Nismo exhaust. Is it plausible that the HFC's caused the CEL's and I'll have to throw on some O2-non foulers?

P0420 OBD-II Trouble Code
Technical Description

Catalyst System Efficiency Below Threshold (Bank 1)
What does that mean?

Basically this means that the oxygen sensor downstream (after) the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

0430 OBD-II Trouble Code
Technical Description

Catalyst System Efficiency Below Threshold (Bank 2)
What does that mean?

Basically this means that the oxygen sensor downstream of the catalytic converter on bank 2 (passenger side) is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Anyone ever had HFC's set off a CEL for the cats not working?

Yeah, mine threw a light a month or so ago. I had a blown out gasket from the header to the cat, Once i replaced it, reset the ECU, it went away and stayed away. Not sure if this will help you or not though. (the gasket swap was a *****) Also, have you been using fuel addtitives regulary? Those are known to clog the filters in the HFC's and envoke cat failure.

No I don't use additives. Maybe I'll try replacing the gaskets though.

DTC troubleshooting for P0420-P0430
 

Take a look here...

http://www.ismcentral.com/dtc/DTC_P0420-P0430.pdf

I extracted these pages for you.

Rick