O2 sensor
 

My 99' 3.8 NA has thrown an o2 sensor malfunction code. Just recently I have been experiencing a slight....pulsating feel on occasion while in idle at traffic lights and such. Pretty sure the two are related. Does anyone know if there is a different code for the O2 sensor before the cat and the one behind? Or has anyone experienced this and could shed some light?

There is a very slight drop in RPMs when the "pulse" happens, maybe 50 or so. It doesn't feel like a full misfire (after two MAF sensors I know how terrifying that feeling is)but it seems like something akin to that. I can hear a faint click or popping noise alongside the feeling in the peddle.

Any thoughts are always appreciated.

If the o2 sensor made a code it was either B1 S1 or B1 S2 with a P0xxx code if it was S1 it was before the cat and S2 is after.