Putting a HF cat (perrin) on my IX. Does the rear O2 sensor go before or after the cat? or does it make a difference?
The stock cat has it right in the middle and there is cat material at both ends?

 

you want the O2 sensor to be as far after the high flow cat as possible. That placement will decrease the chance of the CEL coming on.

 

I have mine after the cat.

 

after

 

+1

 

I'd put on a mechanical o2 fix on there just in case.

 

This^

I put a mechanical fix with my HFC and didn't have a single CEL in over 30K miles on my Evo 8.

 

I would recommend getting rid of the high flow cat and just putting on a test pipe... I had the perrin high flow cat and it held back 33hp of my car and instead of running 11.8's i was running 12.1's. Test pipe it, you will be happier in the long run

 

test pipe ftmfw.

 

33whp is a lot of restriction! Sadly, I live in a state where it wouldn't be a good idea to do that and the class that I want to race in requires a cat.

 

a mil.spec 100 cell cat wouldnt have held back 33hp haha

 

or just shut off the rear o2.

 

Wise words.

The wonders of ECU flashing

 

When I ran a HFC I used a O2 cell delete and you will not throw any codes. Ask any shop or tuner for a cell delete.