Oxygen Sensor

One day the check engine light came on. So the next time I went past an auto place, I stopped and had them plug it in. They said it was saying it was the code, P0141. Which is the oxygen sensor, the one behind the catalyst converter. Today I jacked it up and looked at it. Everything looks find. Except were the covering for the wires meets the sensor, its warn/melted in a way. And around where the sensor is, its like wet, but not actually wet. What could be causing the wetness? The sensor is probably bad but what could possible be causing the wet area? You can see the wet area in the pictures I attached.