Looking for any information about putting a high flow catalytic converter on my V70R.
-Will it set any codes?
-Does the O2 sensor need to be in the Cat or after?

 

older cars used an O2 sensor ahead of the cat, newer cars have a sensor ahead of the cat and another after. The sensor ahead of the cat is the lambda sensor which controls fuel trim, the sensor after the cat just measures the efffectiveness of the cat. Your replacement cat should be appropriate to your model/year. Usually cars that set codes is due to the rear sensor. The usual trick is to insert a CELboss spacer on the rear to move it away from the exhaust stream.