Replacing Air Filter

Replacing the air filter regularly is a necessary part of your car's maintenance. Replacing the air filter with an aftermarket replacement, such as an ITG Foam Filter, or K&N Oiled Gauze filter, can also yield minor performance and efficiency gains.

Time Required:
10 Minutes

Tools Needed:

Flat-Head (slotted) Screwdriver

Pliers or Vise Grips (In some situations)

Spray-On Catalyst/Lubricant (PB B'Laster or WD-40)

Procedure:

The air filter case is a two-part shell with the air filter element between the two halves. The two halves are held together by one screw at each corner (four corners, four screws). The screws are threaded through the upper shell, into metal fasteners on the lower shell.

IN SOME CASES, THE METAL FASTENER CLOSEST TO THE POSITIVE BATTERY TERMINAL WILL BECOME CORRODED BY GASES RELEASED BY THE BATTERY.
If this has happened to you, you will know, because as you try to unscrew the screw closest to the positive battery terminal, the metal fastener will rotate with the screw, and will not allow you to loosen the screw or open the airbox.

To fix this, I highly recommend using a spray-on lubricant on this fastener and letting it sit for 5-10 minutes before attempting to replace your air filter. Personally, I spray all four fasteners with PB B'Laster before servicing my air filter.

If the fastener still turns with the screw, then you need to grip the lower fastener with a pair of pliers or vise grips.

Replacing the air filter

Unscrew all four screws

Separate the top and bottom half of the airbox

Remove the old filter.

Clear any debris out of the bottom of the airbox (leaves, bugs, etc)

Put the new OEM filter in place (Optionally, use an aftermarket filter that was designed for the Focus)

Replace the top part of the airbox

Insert and tighten screws