There is a before and after oxygen sensor setup in my 01 sportage. Without the pre-cat, my chk eng. light would stay on always, but this doesn't hurt the drivability. It still has closed loop data from the primary 02 sensor. the 2nd one is just to check that the cat works. You CAN gut the 2nd cat without it interfering with the ECU so long as you leave the primary. I hear there's a way to make a secondary sensor fooled without having any cats. There's even plug ins for 'race applications only' that go into the wire harness that would otherwise plug into the o2 sensor. I wouldn't let the sensors stop me from buying something like this.
Being that our aftermarket resources are far and few between, I think that we should post the one's we know of on a sticky with categories: engine perf, lifts, body, fan sites, etc. i dunno.. random idea.