I searched around looking for similar problems on the forum but can't seem to find anything. Things to know:

-I have a Koyo radiator in my S4.
-All S4's (not sure about S5's) have the coolant level sensor in the top of the radator.

and from this i am getting a leak. I tried tightening the bolt obviously, but I noticed there is a plastic insert in the bung with the threads on it. This piece now spins freely and the leak comes from OUTSIDE the edges of what looks to be the threads.

has anyone had this happen to them? and anyone have a good fix for it? I'm not sure how to get the sensor out now

 

sounds like you're stripped it.since the rad is made of aluminum and the sensor is made of plastic,it's very easy to strip the threads on the sensor.you should have put some teflon or some sort of thread sealant and been careful.not sure how you could get it out but hopefully the threads that are on the rad are fine.

 

Plug the hole with a proper sized brass plug and remove the sensor. You just have to jump the wire on the connector plug to stop the buzzing. Check the coolant at least once a week. More often if you suspect leakage.

 

you stripped it ... try retapping it .. or try putting on some teflon tape and see if you can rethread it that way