Once a season for an ordinary domestic mower, and mid-season if the ground is dusty or you have cut in drought. It is the cheapest part on the engine and the most neglected.
Why it matters more on a mower than on most engines
A lawnmower’s air intake sits close to the ground, immediately behind a blade that is deliberately throwing dust and clippings into the air. The filter is doing a harder job than the one on almost anything else you own.
A choked element restricts air, the mixture goes rich, and the symptoms are hard starting, black smoke, a mower that runs on when switched off, and no power in exactly the thick grass you bought it for.
The two element types, and the treatment they want
Foam. Washable, in warm soapy water, then squeezed dry — not wrung, which tears it. Most foam elements are meant to be lightly oiled afterwards with engine oil, then squeezed to distribute it. The maker’s manual says whether yours is one of them.
Paper, or paper with a foam pre-cleaner. Not washable, ever. Water collapses the medium and the element never filters properly again. A gentle tap to shed loose dust is the only cleaning, and after that it is a replacement.
Where a paper element has a foam wrapper, the wrapper washes and the paper does not.
Checking it
Take it out and hold it up to the light. Foam that is grey and stiff, or paper you cannot see light through, is finished.
Two mistakes
Running without it. Five minutes without a filter puts more grit into an engine than a season with a dirty one.
Over-oiling a foam element. A soaked filter restricts air as effectively as a blocked one, and the excess is drawn into the intake.
How this site handles it
The air filter shelf here is compatibility-led, because these parts are sold by the engine they fit rather than by the mower. Pages name the engine families the seller names, reproduce the reference numbers as printed, and state whether the element is foam or paper where the listing says.
The questions that keep coming in
Can I wash a paper filter?
No. Washing collapses the paper’s structure and it never filters the same again. Tapping it out gently is the limit, and beyond that it is a replacement.
Does a dirty filter really cost that much?
It richens the mixture, which shows as hard starting, black smoke, running on and poor power in long grass. It is the cheapest part on the machine and the one most often left for three seasons.
Last reviewed 26 September 2026