On most modern vehicles, the air filter is under the hood inside a rectangular cold air collector box that’s located up near the front of the engine compartment. (Other vehicles, including those with carburetors, have big round metal air cleaners that are hard to miss.)
The air cleaner has a large air inlet duct (also called the air intake hose) connected to it. Loosen the hose clamp that seals it to the box, and then undo all the screws, clamps, or wing nuts that hold the lid of the box in place. Put the fasteners you removed somewhere safe so that they don’t roll off into oblivion. Open the lid of the box and . . . voila! . . . you should find the air filter inside (as shown here). Lift out the old filter (it isn’t fastened down) and take a look at it.
Some older vehicles have permanent air filters, and some off-road vehicles have more-complex filters with wet and dry elements. Clean and replace these according to the instructions in your owner’s manual.
To figure out whether your air filter needs to be replaced, just hold it up to the sun or to a strong light. Can you see the light streaming through it? If not, try dropping it lightly, bottom side down, on a hard surface. Doing so should jar some dirt loose. If the filter is still too dirty to see through after you’ve dropped it a few times and it looks as though it just needs a bit of cleaning, you can try to clean it. If that doesn’t work, you need a new one.
To clean a pleated air filter use either an air hose to blow the dirt off (not through) it or a vacuum to suck it out. For both methods, handle the filter gently to avoid crushing the pleats. Keep the nozzle of the air hose or vacuum cleaner several inches away from the filter — don’t jam it up against it. And if you’re using compressed air, do it away from the vehicle to avoid blowing the dirt around under the hood.
If the interior of the box is fouled with dust or sand, before you clean the box paste some duct tape over the open end of the air intake hose so that the dirt can’t get in. Then either use the compressed air hose to blow the dirt out of the box or the vacuum cleaner to suck it out.
When the cleaned filter — or the new one — is in place, put the lid back on the box and replace all the stuff that held it on. Then, remove the duct tape from the open end of the air intake hose and use the hose clamp to reattach it to the box. Done!