Switch fan runs full power when powered off

Hi, i have a Switch with a strange fault.
The Switch was waterdamaged. I cleaned the board and replaced the lcd connector. After assembly when i plug in the battery the fan starts to run at 100 %, but when i boot into horizon the fan works normal. When i fully shutdown the system the fans runs again at 100%.
Do you know what is causing this fault?
I bought the Switch broken.