i switched my civic y7 exhaust to a y8 style, however i have 3 possible spots to attach my o2 sensor. One on the header which I already attached, the down piece of the header and on the high flow cat. My question is does it really matter where i place the sensor or do i have to place it on the cat? thanks!

 

if its the second o2 sensor, it has to be behind the cat.