Looking for O2 sensor locations, codes for
 

I'm having the worst time looking to replace O2 sensors in my 2002 Montero Sport XLS (3.5L). I replaced one downstream a while back with some help but I'm at a loss to figure out which one and the location of both downstream ones. Is there any good diagram or photos out there to show me. A list of common fault codes would be helpful too.

Thanks, Bob.