O2 sensor confusion

Been reading this forum for awhile but this is my first post, hopefully I can finally get this sorted out with your guys help.

I know one of my o2 sensors has failed, but I cannot remember which one it was when I got the error codes checked. Now I was just going to replace them all, but I then remembered that my car has 2x before the cats and 2x after the cats. I have searched everywhere, read countless forums and manuals and nothing will give a definite answer, do I replace just the two before the cats, or all four, and if it is all four, are they the same part or are they different? I read somewhere that the two o2 sensors after the cats are only there to make sure the cats are working properly, but this information was vague at best. I am going absolutely insane trying to sort this out because I would like to just buy two o2 sensors instead of four.



I have a 1994 v6 Mustang, standard transmission. Mechanically it is stock, I have only made aestetic changes like rims, headlight/corner/foglight assembly.

Also which O2 sensors would you recommend, I was looking at Bosch, but I want to get the best ones possible with the hope that I wont have to do this again anytime soon.

Thank you for your help,

Sejin

Unfortunately all of the places around here charge a minimum of $90-120 dollars just to run the code as part of there diagnostic package, so I would prefer to put that money into replacing both O2 sensors. Also the car has about 75000miles on it and I am certain they have never been replaced so if one has failed, the other one is likely to fail shortly after, if not already, since I had the code checked about a year ago.


Sejin