Diagnosis
Weak connection to the VFD (Vacuum Fluorescent Display) or failing display driver. Often a cold solder joint on the PCB.
How to Fix It
Unplug the fan. Press firmly on the plastic panel over the LED display. Do not use excessive force. Plug the fan back in and observe the display. Sometimes pressure reseats a loose connection. Cycle through all functions (speed, timer, oscillation) to see if the dimness is consistent across all segments.
If That Didn't Work
The ribbon cable connecting the display to the main PCB may have a faulty connection. This requires disassembly to inspect and reseat the cable connector. The display itself is a single, non-serviceable unit.
Heads Up
The display brightness is not adjustable. A dim or partial display does not affect fan function, but indicates an impending total display failure.