This is what you need:
So here is my story:
I tried for an hour to figured out a way to take this sensor out without success. Then I read in Pelican's parts section that it is a good idea to change this sensor if it is too old. So I just gave up and I cut the wire to remove it with a regular wrench.
The next day I found the flare nut wrench in a lost corner of my tool cabinet... oh well, another $95.50 to spend for the sensor... Lesson learned.
The best thing to do if you don't have access to this tool, is to use a cheap box or closed wrench and cut a slot in it with a grinder.