I looked around my service manual and it's pretty vivid in perspective in doing just about any job, except it doesnt tell me where my Rear heated 02 sensor is located. Is it before or after the cadillic converter. And how do i test which one out the three are bad. Thanks if you try to help me.

 

What year is the car?

AFAIK, there's only 1 sensor that far back, the others are in the y-pipe.

If you have a code for it, it's probably dead, and that's how you will be able to tell which is bad(via the CEL/SESlight)

 

After the CAT regardless of year, you'll get a code 95-96 it's 0707 code, but it's window dressing anyways so change it when ever. I've had mine bad for over a year

 

http://www.courtesyparts.com/A32_O2-sensors.html