Thomas,
  This happened to me as well. It turned out to be that the new aftermarket coolant sensor had the wrong O ring on it (to small), which caused it to leak, and me to give it another twist with the pliers. Still leaking I took the new sensor out, and put the old O-ring on (visibly larger) and put it back in, no problem. Two weeks later in the middle of nowhere, the new sensor blew its top, leaving the threads in the hole and me stranded. GRRRR! A second FLAPS replacement had the same problem, at which time I went to the dealership and paid twice the price for a clearly superior part, with the right O ring. No leaks, no need to re tighten.

  If possible get a new one from the dealer. I think it was $22 instead of $12. Well worth it.
  Steven

Thomas Pfrommer <pfrommer@PHAS.UBC.CA> wrote:
  Hi,
I discovered today a small leak on my coolant level sensor. I assumed a
bad O-ring and stupidly I wanted to tight it down a bit further until
tomorrow to get new sensor. Well surely another 1/16 of a turn and the
plastic broke and I had to get home via public transport. Now I will get
this part tomorrow (hopefully they have it in stock at VM Authaus in
Vancouver / Burnaby ... a really great place to get reasonably prized
parts the next day by the way .... and a really nice guy .... no I am
not at all involved with this mechanics, its just a good place to get
things fast).
Now I thing when I changed the expansions tank a few months ago it is
very likely, I might have tight the sensor a bit too much so that it
slowly cracked and not the O-ring but an already broken plastic was the
leaking problem in first place. My wrench did the rest then hehe. I
know, next time I should inspect first before tighten it more.
Is there a rule of thumb how to tight down these plastic sensors? How
many (fraction of) turns after it is hand tight? As I have to use a 32mm
socket it is quite easy to over torque it without even knowing ...
Thanks
Thomas

---------------------------------
Never miss a thing.   Make Yahoo your homepage.