Quick answer
On a gas walk-behind mower, the most common no-start causes are stale fuel, a fouled spark plug, a gummed carburetor, or a clogged air filter. On a battery mower, an interlock is a common cause — check the safety interlocks, the start sequence, and the pack before anything else. Work top to bottom below. For any work near the blade or under the deck, disconnect the spark plug wire on a gas mower or remove the battery on a battery mower first.
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 may leave you the recoil cord, if your model has one — if the cord starts it, the starter battery needs a charge, not the engine a repair.
Stale fuel is suspect number one after storage
Gasoline goes stale in storage, and ethanol blends are especially prone to it. 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
Disconnect the spark plug wire, then 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.
If you suspect a flooded engine, follow the flooded-engine procedure in your engine manual.
The carburetor: where old fuel goes to cause trouble
If you have fresh fuel, spark, and air and the engine still cranks without firing, the carburetor jets may be gummed. Two levels of fix:
- Light cleaning. For light varnish, use carburetor cleaner exactly as its label and your engine manual direct, with the spark plug wire disconnected and well away from any spark or flame.
- Bowl-off clean. With the spark plug wire disconnected, remove the float bowl following your engine manual and clean the bowl and main jet. Sticky or crusted deposits that light cleaning won't shift can live here.
If cleaning doesn't hold, consider replacing the carburetor — 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 can prevent starting — check the dipstick, since on engines with that feature a mower that's low on oil may refuse to fire.
Battery mowers: it's the interlocks
A cordless mower with a charged pack that won't respond is often an interlock, not a failure. The usual suspects: the safety key (if your model has one) 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 when too cold and may not run in deep cold; check your battery manual for the temperature range. 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 can point to the ignition coil or a sheared flywheel key (the latter usually after hitting something). Fuel visibly leaking from the carb can mean a stuck float needle, and it is a fire hazard: stop and fix it before starting again. These may be 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.