I installed the second o2 sensor BEFORE the cat on the header, I expected to get a check ingine light but I didn't. TOday i had it placed behind the cat, and replaced it w/ a new sensor. Could it be that my wiring harness is malfunctioning because I didn't get a check engine before? thx!

 

Where is it supposed to go. If it is supposed to go before the cat then the temp after the cat might not be enough to give the right signal.
Make sure the wires aren't touching the exhaust.

 

It's supposed to go after the cat. That's y it's weird, i should of goten a light since the ECU would of thought that i was running too rich or something, the wiring is clean, the harness reached all the way to the new location so I didn't have to cut it. thx