So my car is failing the high speed test for Hydro Carbons by about 20ppm (just barely failing) and its failing the idle test for HC by about 300ppm (standard is like 220ppm, my EX is putting out about 520).

Its passing CO% by about 50% on idle and high speed (really clean).

I am going to try running some 66k from Napa and high octane (91). Than refill it up 1/4 tank.
I got new plugs, wires, cap, rotor, PCV valve, timing (re-adjusted) and everything else.

Seeing how the car has 170k miles, I am really considering replacing the cat.
Should I buy a $400 OEM CAT or a aftermarket Magnaflow CAT?

I just want to pass emissions on this daily beast

 

my opinion, unless your running boost or have a built N/A engine and need the higher flowing one i'd go with the oem replacement but thats just my 2 cents.