How often:
Changing the air filter should be part of any tune-up. Change it every 15,000 miles or once a year. If you live in an especially dusty or dirty area, change it more often.

Air filter test:
Shine a flashlight from the inside out of your air filter. If you can't see any light or very little light, it needs to be replaced. Change the breather filter and check the PCV at the same time. On most cars, this is a fairly simple procedure.

Things you’ll need from your garage or your local NAPA AUTO PARTS Store:
Flat-Head/Phillips screwdrivers
Adjustable or open-end wrench
Pliers
New air filter
New breather filter (if so equipped)

Step 1:
Pop the hood and find the air filter housing.
It will likely be square (on fuel-injected engines) or round (on older carbureted engines) and about 12 in. (30 cm.) in diameter. On older vehicles, it will be in a large round housing on the top of the engine. On newer vehicles, it will be on the end of the large black intake hose opposite the engine mounted on one of the fender wells.

Step 2:
Use a screwdriver to remove the screws or clamps that hold on the top of the housing.
If you have an older vehicle with the round air filter housing, remove the wing nut holding the air filter cover and any other clips.

Step 3:
Take out the old air filter and clean any dirt and debris from the housing with a clean rag.
Be careful that no dirt or debris falls into the carburetor or the intake. Remove the breather filter.

Step 4:
Install the new air and breather filters.
Replace the air cleaner top. Reinstall attaching hardware and air intake hose (if applicable).

Important Tips:
When the air filter is out, you should replace the PCV filter as well. It should only take 5 minutes.