Hey Patrice,
       Had that happen this year to mine.  I tried snugging it down, but that
didn't work.  When I removed the sensor, I found it had a crack in the
plastic, just under the O-ring.  Rather than get a new one, I filled the body
of the sensor with JB Weld, let it sit overnight.  I also went to the
hardware store and got a new O-ring, the old one was hard as a rock
(seriously, it make a 'clink' sound when I dropped it on the table!).  When I
installed it, I also put a thin bead of RTV high-temp silicone around the
O-ring.  That is serious overkill, but I only wanted to do it once for the
rest of my life.  Oh, and don't forget to clean every surface -- a little bit
of antifreeze crust can prevent your O-rings from sealing.