AU coolant sensor replacement

[stuvw]

Hi,
my coolant sensor light (Fairmont) is always on although the reservoir is full.
I guess the sensor is faulty. I assume it is some kind of float?

Any way how do I remove the sensor to check it? - the round black bolock of the right of the tank with 4 holes in it. Do I need a special tool that goes in the holes like the tool for unlocking a circular saw blade to unscrew it?

Or do I have to get a whole new reservoir tank?

Cheers.