Ford Repair: oxygen sensor, emissions test, oxygen sensor

Question
I have to go through emissions and the check engine light came on so i found out it was the Downstream oxygen sensor but before i could replace it the engine light went back off. So my question is do i REALLY have to replace it to pass the emissions test?

Answer
Before you replace it, try to use fuel injector cleaner. Add it to a full tank.  This could help in cleaning the sensor.  As for emmission, I cannot answer if it must do.  As long as the light is out, it means the sensor is working.  However, changing the sensor in this location is very simple task for DIY.

Sam