I had this fault several months ago. It was a customers oven and the cooling fan for the top oven would come on at random times. Sometimes in the middle of the night and stay on for hours.
I was advised to replace the NTC sensor repair set as it had been modified. The part number was 10011733 from Bosch. If you look on the Bosch spares diagram for that model, it is item 1046.
It may be what is at fault, can’t say for definite.
Remember to switch the power off if you are investigating.
Mervin