Hyundai Repair: code p0422, exhaust leaks, hyundai accent

Question
I have a 2001 hyundai accent with 130000 mile ,the check engine light is on  code p0422, I changed both o2 sensors and the front  flex pipe the code keeps comeing back before I get a cat is their something that might be affecting this . The car runs fine??? also I got ahold of a snap-on reader what should the o2 sensor read  and some how it should show me that the cat is bad??? yes /no?

Answer
If you've got no exhaust leaks, you're going to be primarily interested in the amplitude of the oxygen sensor readings when the vehicle is warmed up.  The front oxygen sensor should fluctuate between 0 and 1 Volt with a large amplitude.  The rear oxygen sensor should remain relatively steady around .4 to .6V.  If the rear oxygen sensor fluctuates with an amplitude near that of the front oxygen sensor, the catalyst needs to be replaced.  In this case, it'll be the catalyst attached to the exhaust manifold, since that's the one between the two converters.