ok my car has 155k but the motor has been completely rebuilt...Should replace/clean my o2 sensors?

 

wouldn't be a bad idea to REPLACE...o2 sensors cannot be disassembled and cleaned

 

should i replace both before and after the cat?
Also would changing my cat from a stock to a magnoflo 2.3 help improve my mpg?
I checked out pep boys and the have univ and oem type. I guess oem type right? or maybe oem for before the cat and univ for after?

the whole thing here is to help restore mpg

 

bump

 

replace them both....and try to get a cat w/ the o2 bung already in it since you're obd2 and will need this...unless you buy a universal performance one and weld a bung in for the rear o2 sensor