Cooling Fluid Level Sensor Troubleshooting

Trouble:
Making the very first checking of the levels I came to notice that the header tank on top of the radiator was dry-empty and so there was possible a lack of fluid. But the light supposed to warn me of such problem did not operate on the fascia.
I topped up and needed more than 1,5 litres until I could see liquid through the header tank fill opening.
But still remains the problem of the electric level check. So my purpose is to make the coolant level warning work properly.
Do the warning lights work ? The "fuel button" check mentioned in the owner's HB p63 does strictly nothing regarding the lights. It is supposed to light three lights : "accumulator, coolant and fuel level" if pressed, ignition being switched on.
But this whole set of warning lights does not seem to work under this check. Although "low fuel" and "parking brake" do operate properly.
To check:
1- the wires to the sensor are in pretty bad condition (rusted). Is there any electricity reaching this element ?
2- the bulbs behind the fascia [They do work for when I switch the ignition on all of them briefly flash]

About Capacitive Sensors A Video explains the process here

About the level sensor (from Brian, RR Forum) :
Jean-Christophe,
While your wiring could be a problem, two others are more likely when the coolant level lamp is misbehaving. (This is presuming, of course, that your coolant level is not, in fact, too low.)
1. Over the decades the tips (French : pointe de la sonde) of the probes in the tank (see picture) can slowly get corroded or just collect crud such that they are effectively insulated. If that occurs they can't serve their purpose. It's a fairly simple matter to remove them and clean up the tips. Sometimes their insulation gets pretty bad as well. If that's the case then strip them bare, clean them up, then use heat shrink tube insulation to re-insulate all but a short bit at the end that dips into the coolant.
2. Some of the electronics in the coolant level amplifier have a tendency to fail with age. They can be repaired. When I get back to my computer that has the article I wrote on repairing these on it I'll post a link. It's probably already posted in the Shadow forum as well. Search on coolant amplifier. Article on coolant level sensor amplifier

About changing thermostat
Hi Jean Christophe

If you are planning on changing your coolant, it will be a great time to check/change your thermostat. They retail at about $120, however there is another type that will fit with slight modification, the stant 13558.
This retails at around $25. Check out this document, co-authored by Brian, for further info.
Article on thermostat in general and Stant

About checking the hoses
Question
Is there a technique to check the hoses condition from the inside (after removing them) or would it be better to forget about it and change them along with the thermostat.

Reply
I don't know the definitive answer to that question. In my case, when I checked and flushed my cooling system I could see the hoses were flexible, had no cracking and generally looked in good condition so I did not change them.
That was about 2 years ago and they have not given any trouble.