Okay so my 97 RS has been needing o2s for a long time, I'm replacing the front two o2 sensors and the CPS tomorrow but have some questions.

Does the check engine light trip a closed loop fuel (Rich fuel trim) for any code?
Say not having the rear o2?

 

ya the pcm will defiantly throw a code for not having rear o2s, something like p0420-catalyst system efficiency below threshold.

 

But does the check engine light throw it into a rich closed loop or does it only do that for front o2s?