Under normal driving conditions, Mazda recommends replacing your Mazda 3's engine air filter once every 30 months or 37,500 miles--whichever comes first. If you drive on rough or dusty roads, or in a humid climate, consider replacing the filter every 15,000 miles. At first glance, the Mazda 3's air filter is not visible. The car does have one, it's just buried underneath several engine compartment covers and panels. Expect the replacement procedure to take 15 to 20 minutes.
Shut the engine off. Open the bonnet. Inspect the driver's side of the engine compartment. There are two plastic covers: one in the front and one in the back. The first cover is the duct; the second cover is the battery cover. The air filter compartment is beneath the air-duct cover.
Remove the two plastic screws from the outboard side of the air-duct cover. Depress the clip on the battery cover to lift it from the battery compartment. This frees the air-duct cover. Slide the air-duct cover toward the front of the engine compartment to remove it. You can reattach the battery cover now, or you can wait until you're finished.
Remove the plastic screw from the panel beneath the air-duct cover. Grasp the panel and pull it out of the engine compartment. This provides you with access to the air filter compartment.
Unbuckle the two clips on the outboard side of the air filter compartment. Lift the top of the compartment to view the filter. Lift the filter out of the compartment.
Lift the filter compartment cover again and insert the new air filter. Make sure the filter's rim is facing upward. Close the filter compartment cover. Buckle the two clips.
Position the panel back in place. Replace the screw to secure it to the engine compartment. Reattach the air duct. Position it in place. Lift the top of the battery cover and insert the end of the duct into the battery compartment. Tighten the two screws to finish.