Genuine Daikin 4000835 Indoor Air Filter is the washable intake filter that sits behind the front grille of your wall-split, catching dust and lint before air passes over the indoor coil. As it loads up with dust, airflow from the unit weakens, the fan works harder, and cooling or heating slows. Supplied as a single genuine filter cut to the unit's exact frame, it seats properly and lets the unit breathe — where a trimmed generic can leave gaps that bypass the coil.