The air filter on a lawn mower is responsible for allowing clean air into the engine. The filter does two things - it lets the right amount of air in to mix with the fuel in the engine, and it keeps out dust and dirt particles that could prevent the mower from running efficiently and could also cause internal engine damage. The air filter should be cleaned or replaced regularly. If it gets clogged up it can prevent enough air from entering the engine, which may cause starting or running problems.
Replacing the air filter on a Toro 22" Personal Pace Recycler Mower (model #20332) is an easy task that just about any owner can do themselves. The only things you'll need are the new air filter (part #491588s) and a flathead screwdriver. It should only take a few minutes to install the new air filter.
As you're standing in front of the lawn mower, the air filter is located on the left side of the engine. It is a flat air filter, covered by a black plastic cover. There's one hex head screw holding the cover on. This screw is located in the top middle area of the air filter cover; remove it with your flathead screwdriver.
With the screw out, you can remove the cover by pulling it out from the top. The air filter will now be exposed - a flat style air filter made up of pleated paper. Remove this filter, and if you have a rag or paper towel handy, wipe down the air filter housing to remove any dust or debris. It's also a good idea to wipe down the air filter cover.
Now install the new air filter, setting it into the housing so that the pleats face outward, away from the engine. The air filter should fit in snugly. If it doesn't fit right or it's not the same size as the old air filter, you may have the wrong model filter. Never install an air filter that isn't specifically designed for your mower.
Once the filter is in place, reattach the cover, making sure the screw hole lines up with the hole in the engine frame. Then put the screw back in and tighten it so the cover does not move around at all. Your new air filter is now installed.
(Originally published in similar form on Helium.com, July 2012)