The air filter on the Ford Focus MK2 is required to be changed every 12,000 miles or every year (basically every time the Ford Focus is serviced.)
You can easily replace the Air Filter in the Focus yourself using just a Torx screwdriver, and a couple minuets of time.
Replace Ford Focus Air Filter
Open the bonnet on your Focus, and locate the Air Filter housing (Pictured above). This engine bay is from a 2.0l Duratec HE (Petrol), other Ford Focus MK2 engine bays may differ, but they are all laid out similar.
Locate the 6 Torx screws holding the top of the box in place. Note the one screw under the piping can be a little tricky to get into. If this is the case, the airbox simply pulls out towards you, which will allow you to twist the air box to gain access to that one hard to reach screw.
These screws do not come all the way out, which is a great idea, as it stops you and technicians losing screws down into the engine bay when it comes time to service the car and replace the air filter.
Once the screws have been removed, the air box lid simply pulls up, revealing the air filter – The picture above shows a K&N Reusable Air Filter – These are simply just washed using water, and then re-lubricated using special oil.