[Solved] Audi TT MK1 225KM - The temperature is going crazy, the fans are not turning on

Hello colleagues.
The problem seems to be complex and I hesitated between starting this topic in the heating, mechanics section and this one. I chose this section due to the fact that the end symptom concerns the electronics in the car.

Symptoms:
1. The car does not heat up even during very hard driving. The temperature on the indicator on the board and channel C49 on the climatronic shows about 64 degrees.

2. The car overheats when stationary and the fans do not turn on. Temperature checked at the same time in 4 places:
a) 90 degrees on the pointers on the board,
b) 96 degrees on channel C49 climatronic
c) 100 degrees on the OBD2 reader
d) 114 degrees measured with a laser thermometer on the pipes at the temperature sensor

3. The OBD2 scanner does not show any errors.

Replaced thermostat (opens properly at about 80 degrees). The coolant temperature sensor has been replaced with a new one but a replacement.
Checked:
both fans work on both speeds when connected briefly to the battery
a voltage of about 5V is fed to the temperature sensor
the fans are turned on as in the plug from the temperature sensor a jumper is in

Any sensible hint of the importance of gold in this case. Thanks for showing interest!

Water pump ineffective at low engine speed.

Added after 3 [minutes]:

It is supposed to hold about 90 degrees.

We tested the car while driving at high speed 5-7k for some 15 minutes in 30 degrees heat. The temperature on the pointer and on the climatronic only finally reached 64 degrees.

The fact that the fans run briefly does not mean that both gears are controlled by the car.
Are the fans running after the air conditioning is turned on?
From controlling the fans, there should be a fan controller here and it is controlled by the thermo-switch on the cooler. See if you have a thermocouple on the radiator and if the fans will work after bridging its contacts - unless it is not there and when the fans are to turn on, says ECU.
Now I don't remember exactly, but I think the first fan speed turns on above 100 degrees.
The thermostat should open a little later, unless otherwise stated here.

thanks to Grala1 with the answer

Well, both gears are not controlled by the car, and they should be if everything worked properly.
The radiator fans do not work when the air conditioning is on. The air blows with great force, but only with warm air. What does this say?
The fan driver does not know where it is. If someone can suggest it, we will save a lot of time searching.
We got the Thermowell, but it's hard to get to him and today there was no time because it was dark. Ok, I will bridge it as you say and let you know what the result.
1st gear 94 2nd 104
The thermostat somehow above 80 degrees opened, probably close to 85, I don't know if it's good or bad ...
Overall, I slowly lose my hopes for self-repair ...

See the left stringer.
The first gear is usually controlled directly from the thermo-switch in terms of engine cooling, the second is already controlled by the module.
When you turn on the air conditioning, the first gear should turn on through the module.
Maybe there is no factor so that the compressor and fans do not turn on.
You must check that the thermocouple is giving a signal.
There should be a 3-pin cube and one constant plus in it and after giving it to the rest, the appropriate fan speeds should start.
Maybe a fuse from the power supply of the fans blasted you and here is the problem.
On the first and third pins (probably described as 30a and 30b) of the 4-pin plug of the fan controller there should be a constant plus.
I read that the jumper works, but check the power supply I wrote about.

Exactly as "grala1" writes, under the car and on the left side member there is a fan and air conditioning module, it is very often flooded or has rotten wires.
Let you know if you understand switching on the air conditioning by pressing the button only or whether the compressor clutch is physically engaged?

The problem seems to be solved. It was the thermowell, as my colleagues suggested above.
Now the fans turn on in first gear already at 70 degrees on the pointer. Unfortunately, the problem with the car not heating up while driving is still valid. At a standstill, the temperature after about 20 minutes of engine running reaches 70 degrees, and then 5 minutes of driving is enough and drops to 60 degrees.

The thermostat was to blame again. We replaced it with a brand new one and everything flashes.

The thermostat was to blame again. We replaced it with a brand new one and everything flashes.

Added after 2 [minutes]:

All issues resolved as per the posts of this thread.