Axial Front fan for the Ultimaker 3 series and the S Series.
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.
Please see attached link for recommended steps to replace the fan.