I understand the O2 sensor is between the exhaust manifold and the C. Converter. I can't seem to locate mine. I did find a bolt/plug that was screwed into the exhaust pipe in the position I would think the O2 sensor should be...I have a 1991 4.0L man. tranny. is it possible that someone had removed it ? will the motor still run as it should without it ? Mine seems to run pretty well, but maybe not as good as it should? Any ideas ?

 

It will run rich without the sensor, it's located on the down pipe just below the exhaust manifold.

 

You can see it behind and above the yoke.

 

yep, that is where mine is supposed to be. I think someone prior to me removed it for some reason. I have a Bung with a plug there and not a sensor. Also, I noticed a plug up near the charcoal box in the engine bay on the pass. side. is that maybe where the O2 Sensor would connect ? I will get one and install it and maybe it will help my mileage.