So I broke out the ole SCT X3 yesterday and decided to try out the datalog feature (which is bad@$$) on it since the 'stang has been running crappy lately. So i get it all hooked up the the laptop and datalog about 24 parameters, a few of which being the O2 sensor voltages. I noticed the front 2 seem to be functioning correctly. One of the rear ones was lathargic, it did not have anywhere near the changes in readings like the front 2 had. And the other rear one was deadlined the whole time.

I'm no mechanic by any means but it seems like the rear two need to be replaced. Or is it common for the rear to not have as many changes like the fronts have? Also, if the one is dead lining like that shouldn't the CEL come on? or because there's no voltage it doesn't read a problem?

Thanks for your help/insight in advance.