Opel astra h - Problem with the cooling fan from the liquid cooler

Hello !
I have a problem with the radiator fan in the Astra h 1.9 100km.
The problem is that the radiator fan does not turn on at all. I checked whether the fan is working, whether the relays are working, whether there is voltage, everything Ok. I decided to replace the thermostat because at 90 * the bottom of the radiator was cold. After replacing Ok - the cooler resisted heat, while the radiator fan turns on only when the air conditioning button is pressed, where can the problem lie? How does this relate to and what could be to blame for it?

There are a few basic principles to the most common cooling system:
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The system is pressurized, which means that everything must be tight, because only then the liquid (water) boils at 120'C, otherwise it will not work.
The thermostat opens at 88 ° C causing the fluid exchange between the engine circuit and the radiator, thus this temperature is assumed to be optimal for the engine operation.
The temperature sensor that activates the fan will work at 94, and sometimes also from a higher 96-98'C, so if there is no pressure, the water will simply boil and the fan will stand still.
The rubber hoses that connect the radiator to the engine under internal pressure must be hard.
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This is a classic.

It must be remembered that there are many other systems, additional measurements, etc.
In sum:
- either a blown temperature sensor activating the fan, or no power supply at some point in this circuit, or no pressure in the cooling system.