On a cat back system, should the o2 censor not be behind the cat?
 

Alright, so I have a 2002 Honda Civic with a D17a2, Megan racing headers and exhaust. I live in Georgia, and we have emmisions here. So my car has been throwing a code, P0420, and its causing me to not be able to pass emmsions. It has new Cat in it, and its still throwing the code. I put an 02 Spacer to remove the sensor out of direct airflow, same effect. Then I realized, The 02 spacer was in front of the Cat! Should it not be behind it?!?! I felt so stupid when I realized that. So what I was thinking about doing is cutting the cat out, cutting a section of the exhaust out in front of the 02 sensor, and putting a flex pipe where the cat originally was. Just so everyone knows, I did not put the Cat there, a mechanic did... Fail... What do you guys think?

Clarification:
Cutting out the section of pipe where the Cat is, then fill that gap with a flex pipe. Then, cutting another section of pipe in front of the 02 spacer, and put the cat there.

^^agreed. Honstely, I dont know too much about emissions since I live in Iowa but it seems like Sizzlinnninja's solution should work. Do you have 2 sensors, one before the 02 sensor and one after? If so the only thing I can think of is having the 2nd 02 sensor actually on the cat.