Where is the downstream O2 sensor? I found two in the engine compartment, one so close to the firewall you practically have to pull the engine to get to it, and the other up front... is there a third?

 

Assuming you've got a V6, there should be one around the catalytic converter.

 

yes, it is a V6... I have run my hand all along the exhaust and have not felt it... if I had a lift I would look for it, but alas, no lift... I have begun to believe that it does not have one.

 

As I understand it, there should be one O2 sensor for each exhaust manifold. And those are the ones I think you've found. The "values" that these generate are compared to the "value" generated by another O2 sensor near the outlet of the catalytic converter to determine if the exhaust has been cleaned up as well as the richness or leanness of the exhaust gasses.

Unless you've lifted the car off the ground, I doubt you'll be able to reach it, let alone see it because the ones I've seen on other vehicles have been mounted between the exhaust pipe and the floor pan. You'd have to have really, really long arms and fingers.