Do you have to have the back O2 sensors in the exhaust for MIL eliminators to work? My car just has bolts in the H-pipe where the back two O2's go. If so do I even need the back two to work right or can I just throw some used ones in or somethin?
Used ones will work fine. The O2 sims trick the computer into thinking everything is fine back there. I don't see why you just couldn't attach the sims and be done with it but I don't remember if the O2's act as a closed loop or open.
The rear sensors are nothing actuallly catalyst efficiency monitors. In other words, they send a signal to the PCM telling it whether or not the catalyst is doing its job correctly or not. If not, it illuminates the MIL. If so, it's business as usual.
The rears have no bearing on A/F ratio, etc. You don't need the rear sensors to be in there at all. Lots of people just eliminate them.