The system fans are positioned at the back of the Ultimaker S3 and Ultimaker S5. They ensure the electronics and power supply are sufficiently cooled during printing.
It is recommended to check these fans once a year and clean them if necessary. If the printer is in a dusty environment, this may gather on the blades of the fans.
Caution: Before you start, ensure the fans are not spinning. Ensure the build plate and print cores have cooled down and turn off the printer.
Use a moist cotton swab to reach the fans' blades behind the fan guards. Alternatively, simply blow into the fan or use an air compressor.