Ultimaker Axial Fan 35x35x10

The axial fan at the front of the print head spins when one of the print cores is above 40 ºC. This helps to prevent the heat from the nozzle traveling too far upwards. Without this fan, the filament could get soft above the heater block, which leads to extrusion and print quality issues.

Check there is no Debris preventing the fan from spinning. Instructions here

If the fan is not spinning, it is recommended to replace it before starting a new print. The following steps describe how to correctly replace the front axial fan.

Please see the following link for replacement instructions.
https://support.ultimaker.com/hc/en-us/article_attachments/360010303219/Ultimaker3-Repair-Manual-Axial-Fan.pdf