What is that plug under the exhaust manifold?

[FortuneKouki420]

My ECU is reading a rear o2 sensor circuit malfunction and I can't find the rear o2 sensor on my cat. My car is 95, ODB2 and I initially thought it was ODB1 so I changed the front o2 sensor but the engine light came back on. Can someone tell me what that plug is? I checked the FSM but without success.

http://i1124.photobucket.com/albums/l569/koukimonster36/plug1.jpg

http://i1124.photobucket.com/albums/l569/koukimonster36/plug2.jpg

http://i1124.photobucket.com/albums/l569/koukimonster36/rearo2.jpg
(burrowed this picture)

I was at an autobody at first and the guy told me that plug is the rear o2 sensor.. I think he's bullshitting.. because the rear o2 should be right there but I can't find the slot after my cat! WTF. I can't find the rear o2 wire either. Thanks.

[silver350z05rb]

The last picture you posted,is the 2nd sensor,not sure why you are missing it

[rcdad123]

OBD 1 only have 1 o2 sensor. obd 2 have 2. one on the manifold and one on or after the cat. the plug you were looking at is for air injection.

[mikerbike]

Your first two pics show an S13 exhaust manifold and downpipe. The last picture is an S14 downpipe/cat.

S13 only have one O2 sensor. So if those two first pics are your S14, you need to find another manifold/downpipe, or weld another O2 bung on.