Display shows CF after a period of operation; unit still works but prompts maintenance.

CF is usually not a fault but a reminder.

Steps:

• Remove filter: Open the front or rear grille and slide out the air filter(s).
• Clean filter: Vacuum dust off, then wash with lukewarm water and mild detergent. Rinse thoroughly and let dry completely.
• Reinstall: Insert the dry filter back into the unit.
• Reset reminder: Press and hold the designated filter reset button (often the "Filter" or "Timer" button) for several seconds as described in the Klarstein manual until CF clears.