I have a non-California emission lancer and the catalytic converter is damaged (rusted thru and broke away from the tail pipe). I have a replacement Cat and one of the o2 sensors has been throwing a code so I was going to replace both while im at it. I can not figure out which sensors to get. I ordered a Walker upstream and Downstream and the new upstream has a female end on it. What sensor is the one upstream of the Cat and which one goes into the Cat? some places show intermediate and center even for non CA emission.
Thanks

 

Why replace both sensor when one is faulty?
Replace the faulty one only
As a general rule of thumb, if the front O2 sensor carks it, put in a new one
If the rear one carks it, get one off a wreck and put it in

Reason being as these sensors get older they are not as accurate as new
Think of them as giving.your car a nose so it can sniff the exhaust
The front and rear O2 sensor voltage is compared by the ECU
If the voltage is too close it will throw a CEL
Hence why it's a good does for new one at the front and an old one at the rear.
Check out your local Wreckers