Thank you for the picture.
So, the display is changing from -19 to -23 and then back to -19 and so on, like a loop?
Is that correct?
If it is, then I would say the button that is used to change set temperature is stuck close.
Is that the yellow button near the thermometer
icon or is it the -/+ icon?
Which ever button used to change the set temperature, try to push it again and feel if there is a feeling of a contact getting close or if you can compare the feeling to other buttons on the panel and see if there is a difference.
You can also try to use an adhesive tape, use it to pull the surface of the button hoping that the contact would open up.
Observe if the display woulds top changing.
The electronic controller thinks that the button is being pressed that is why the display is changing always.