Freezer fan problem.

Hi,
I have just cleared a load of ice from my liebherr freezer evaporator, I had lost the contents of the freezer as it wasn’t working properly. Having cleaned out the ice, I checked the fan which is working on the bench, but it doesn’t seem to be working when I switch on the freezer.
Does the fan run all the time from power on, or does it switch in at a certain temperature, or do I need to replace the thermostat or control board?

Thanks.