I just did my emissions and it failed, hc was way high and failed, co was way low and passed. I check the oxygen sensor and it was disconnected, but when i connected it, it ran really rough, disconnected it again, it idled smooth. What does this tell me? if anything, the o2 sensor must work or it would cause the engine to idle rough.

 

It could be that the O2 sensor is bad and whoever had the car last didn't feel like fixing it. Try replacing it and seeing is it helps. Alternatively, if you have a scantool you can see if the O2 is bad before replacing it.