Subaru 2003 Legacy - Manual (Page 184)

4-22
Air filtration system (if equipped)
If your vehicle’s air conditioning system is equipped with a optional air
filtration system, replace the filter element according to the replacement
schedule shown below. This schedule should be followed to maintain the
filter’s dust collection ability. Under extremely dusty conditions, the filter
should be replaced more frequently. Since the filter element is a viscous
type, it is unnecessary to clean or wash the element. Have your filter
checked or replaced by your SUBARU dealer.
Replacement schedule:
Every 12 months or 9,300 miles (15,000 km) whichever comes first
NOTE
The filter can influence the air conditioning, heating and defroster
performance if not properly maintained.