1999 Oldsmobile Intrigue O2 sensor

1999 Oldsmobile Intrigue 6 cyl Two Wheel Drive

I am replaceing the cat converter and the O2 sensors the rear O2 sensor is no problem I have a direct fit so am replaceing it from exaust manifold to the exaust where is the pre O2 sensor it is not located on the pipe which is what everyone keeps telling me I have cat con off and was not on the old and no place on the new but they swear it has one