i got a 08 coupe 2.5

im getting p0420. I checked live data on my 02 sensors.

s1 im getting 2.0 -- 2.3v
s2 im getting 0.5 -- 0.7v
catemp1 is 600f

i also check the ohms on my sensor 1, looks fine. im just thinking it is the Cats. not sure which one though. any thoughts?

Unless you run on ethanol heavy gas and have like 300K on the engine, I think it might be a bit premature for cat failure. Never-the-less, you have a code and might as well proceed with the recommended diagnoses. Look here at page EC-278:
http://www.nicoclub.com/service-manual? ... ma/2008/ec

Checking for exhaust leaks is in order. Checking air intake leaks as well. Checking ignition timing and idle speed are recommended. Then ignition coils, then spark plugs, and finally injectors.
Actually, there's quit a bit to review before replacing a cat.

I recommend working through the steps as best you can. Please let us know how it goes.