Quick answer
On a gas walk-behind mower, the overwhelming majority of no-starts trace to four things, in this order: stale fuel, a fouled spark plug, a gummed carburetor, or a clogged air filter. On a battery mower, it's almost never the motor — check the safety interlocks, the start sequence, and the pack before anything else. Work top to bottom below; most mowers are running again within the hour.
First, the ten-second checks
Before touching a tool: is there fresh fuel in the tank, is the bail lever (the safety bar on the handle) pulled fully to the handle, and is the spark plug wire actually seated on the plug? A boot knocked loose during storage or transport is a surprisingly common "dead" mower. On electric-start models, a flat starter battery still usually leaves you the recoil cord — if the cord starts it, the starter battery needs a charge, not the engine a repair.
Stale fuel is suspect number one after storage
Ethanol-blend gasoline starts going off in as little as 30 days. Fuel that sat in the tank over winter oxidizes and leaves gum and varnish through the fuel system, and the mower pays the price in spring.
If the gas in the tank is from last season, drain it (a fluid siphon or the carb bowl drain beats tipping the mower), refill with fresh fuel, and try again. If the mower fires briefly on fresh fuel and dies, the old fuel has likely already varnished the carburetor — keep reading.
Pull the spark plug and look at it
Remove the plug with a plug socket and inspect the tip. Heavy black soot, oily wet deposits, a cracked porcelain insulator, or a visibly worn electrode all mean the same thing: replace it. Plugs are cheap enough that replacement beats cleaning, and a marginal plug can pass a visual check and still misfire under compression.
While the plug is out, this is also the moment to clear a flooded engine — if you smell raw gas after repeated pulls, let the cylinder air out a few minutes with the plug removed before reinstalling.
The carburetor: where old fuel goes to cause trouble
If you have fresh fuel, spark, and air and the engine still cranks without firing — or starts only on starter fluid and dies — the carburetor jets are gummed. Two levels of fix:
- Spray clean. With the air filter off, a carb/choke cleaner sprayed into the intake and through the bowl drain clears light varnish. Many post-winter mowers come back with just this.
- Bowl-off clean. Drop the float bowl (one nut underneath), spray the bowl, the nut's metering hole, and the main jet, and clear the jet with a strand of wire. Sticky or crusted deposits that spray alone won't shift usually live here.
If cleaning doesn't hold, carburetor replacement is usually cheaper than a shop rebuild on a walk-behind — match the carb to your engine model number, not the mower model.
Air filter and the rest of the checklist
A filter packed with debris chokes the engine at start. Paper elements get replaced; foam pre-cleaners get washed, dried, and lightly re-oiled if the manual calls for it. Two more that catch people out: a fuel filter or fuel line blocked with debris (the inline filter is a cheap swap), and on many modern engines a low-oil sensor that silently prevents starting — check the dipstick, since a mower that's low on oil may refuse to fire by design.
Battery mowers: it's the interlocks
A cordless mower with a charged pack that won't respond is nearly always an interlock, not a failure. The usual suspects: the safety key or bolt not fully inserted, the handle not extended and locked into its detents, the battery door not fully latched, and the two-step start (hold the button, then pull the bail) done out of order. After that, verify the pack in another tool from the same platform and check the LED for an overload or overheat code — we cover the platform-specific patterns in the EGO mower guide and the Ryobi 40V mower guide.
One seasonal note: lithium-ion packs refuse to charge below roughly 32°F and won't run in deep cold. A "dead" battery mower on the first warm weekend of spring is often just a cold pack that lived in the garage — this is a protection interlock, not a defect.
When it's not a driveway fix
No spark at the plug with a known-good plug points to the ignition coil or a sheared flywheel key (the latter usually after hitting something). Strong fuel smell with fuel visibly leaking from the carb means a stuck float needle. Both are fixable at home with basic tools, but if the mower also lost compression — the cord pulls with no resistance — it's time to weigh repair cost against replacement.