HMS, here are thethings that they do . My first o2 sensor still working. But for my 2nd o2, will my OBD-scanner are saying the circuit not functioning. My best bet is that they had somehow dislocate the 2nd o2. Anyhow, the 2nd O2 are not that important. You can have it discon.Basically the 2nd o2 just detect whether the emission that your cat giving out are according the the environemt spec. So here is what happen. should a 2nd O2 detect your engine are giving our a lot of unhealthy smoke, it will give signal to the ECU to be increase the air in. That way, you can get a cleaner air. Since I am not a good environmental ppl, I really don't care how poison my emission is. So F it.
As far as the CEL light, is still working. No probelm. I have my car scan by OBDII scanner and all are in good shape except the 2nd O2 sensor.