Genuine Daikin 4017130 Photocatalyst Filter is the photocatalytic filter that sits in the indoor unit's airflow, its coated media working in light to break down odour compounds as air passes through. As the media ages it loses effectiveness and can restrict airflow, so odours linger and the unit moves less air. The genuine photocatalyst filter carries the correct coating and frame for the unit, working as designed where a generic can't match it.