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Last updated October 28, 2026 · Applies to: lawn mowers from Toro, Husqvarna, Honda, Troy-Bilt, John Deere and Craftsman and others
Cause 1: Stale or contaminated fuel
Step-by-step instructions for this cause are being written.
Fuel path: tank to cylinder. Fuel must flow through every stage. A blockage at any box starves the engine.
Cause 2: Clogged carburetor or main jet
Step-by-step instructions for this cause are being written.
Inside a float-bowl carburetor. Varnish from old fuel usually blocks the main jet or emulsion tube first.
Cause 3: Fouled or failed spark plug
Step-by-step instructions for this cause are being written.
Ignition path: magnet to spark. Test the end of the chain first: a spark tester on the plug lead.
Cause 4: Dirty or oil-soaked air filter
Step-by-step instructions for this cause are being written.
Air intake path. A restricted filter richens the mixture; a leak after the carburetor leans it out.
Cause 5: Primer bulb or choke not working
Step-by-step instructions for this cause are being written.
Primer and choke circuit. A cracked primer bulb pushes air, not fuel. The choke must open once the engine fires.
Cause 6: Flooded engine
Step-by-step instructions for this cause are being written.
Cause 7: Blade-brake bail cable or safety switch
Step-by-step instructions for this cause are being written.
Blade-brake bail and safety stop. Releasing the bail grounds the ignition and brakes the blade. A stretched cable keeps it grounded.
Cause 8: Sheared flywheel key
Step-by-step instructions for this cause are being written.
Ignition path: magnet to spark. Test the end of the chain first: a spark tester on the plug lead.
Which cause is it?
The decision table is being compiled from shop pricing and manufacturer data.
Brand notes
Brand-specific notes for Toro, Husqvarna, Honda and Troy-Bilt are being researched.
When to take it to a shop
Hand it over if the engine still will not run after the fuel, spark and air checks, if you find metal in the oil or the
engine is seized, or if the machine is under warranty (opening it yourself can void it).
Find a lawn mower repair shop near you
Use your location to jump to the nearest city we cover, or pick your state.
Answers being written: Why won't my lawn mower start after sitting? · Can a bad spark plug stop a mower from starting? · How much does a shop charge to fix a mower that won't start? · Should I use starting fluid on a lawn mower?