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Lawn mower won't start

First pull of the season and nothing? Take a breath — this is the most common small-engine complaint in the country, and the usual fix costs less than a jerry can of fresh gas.

tuneWhat are you noticing?
Select anything that matches and the checklist below reorders to fit. Not sure? Skip it.
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In Canada, stale gas is the first thing to rule out on a mower that sat over winter. Open the fuel cap and smell: fresh gas is sharp; stale gas smells sour, like varnish. If it sat since fall, don't agonize — drain it and refill with fresh fuel plus stabilizer. It's the most common cause by a wide margin, and the fix costs a few dollars.

Ranked from real-world failure data. The order updates as you select what you’re noticing above.

Likely causes, in the order to check them

Showing the typical order
Beginner-friendly Doable, with care Safety-critical Call a pro

visibilityHow to check

  1. Open the fuel cap and smell. Fresh gas smells sharp; stale gas smells sour — like varnish or old paint thinner.
  2. Look at it in the light if you can see into the tank: stale gas runs darker, and may look cloudy or separated.
  3. If it has sat since fall, treat it as stale even if it smells borderline. Draining and refilling costs less than any part you'd replace by guessing.
buildNo tools schedule5 min

handymanHow to fix it

warning Fuel work happens outsideGasoline vapour is heavier than air and finds ignition sources you forgot about — a water-heater pilot, a garage heater, a cigarette. Drain and pour fuel outdoors only, engine cold, with nothing burning nearby, and wipe up drips before you pull the cord. If draining the tank feels like more than you signed up for, a small-engine shop will do it as part of a spring tune-up.

Drain the old gas outdoors into an approved container, refill with fresh fuel, and add stabilizer to every can of small-engine gas from now on.

  1. Work outdoors with the engine cold. Siphon or drain the tank into an approved fuel container — many mowers also have a carburetor bowl drain screw underneath that lets the last of it out.
  2. Refill with fresh gas. Buy small amounts you'll use within the season, and add fuel stabilizer to the can when you fill it.
  3. Take stale gas to your municipal household-hazardous-waste depot — not the drain, not the ground.
  4. Prime or choke per your manual and try again. If it still won't fire after the fresh fuel has had a chance to reach the engine, move to the spark plug.
schedule30 min savings$5–20
Fuel stabilizerAny major brand; add to every can of small-engine gas at fill-up

visibilityHow to check

  1. Walk-behind: pull the bail lever fully against the handle and hold it there for the whole starting pull — half-held doesn't count.
  2. Make sure the blade drive (if yours has a separate blade lever or knob) is disengaged. Most mowers refuse to start with the blade engaged.
  3. Riders: sit fully in the seat, set the parking brake, put the deck in the disengaged position, and follow your model's start position for the throttle. Any one of those switches can silently veto the start.
buildNo tools schedule5 min

visibilityHow to check

report Pull the plug wire before anything elseA mower blade turned by hand can spin the engine enough to fire it — with your fingers in the way. Before any work near the deck or blade, for any reason, pull the boot off the spark plug and tuck it away from the plug. Two seconds, every time; it's the rule that keeps hands attached. And if you tip the mower to look underneath, keep the carburetor and air-filter side facing up so fuel and oil stay where they belong.
  1. Check the obvious first: press the rubber boot firmly onto the plug. Wires get knocked loose in storage, and a half-seated boot means no spark.
  2. If it still won't fire, pull the boot off and remove the plug with a plug wrench or deep socket (commonly 3/4 in. or 13/16 in.).
  3. Read the tip. Light tan-grey means healthy. Sooty black means fouled. Wet with gas means flooded — leave the plug out for 15 minutes to dry, and don't crank it while you wait.
  4. Check the gap against your engine manual (many mower plugs run near 0.76 mm / 0.030 in.) and compare the electrode for rounding or heavy wear. When in doubt, a new plug is $6–12 and removes the question.
buildspark plug wrench schedule15 min

handymanHow to fix it

Replace a fouled plug with the exact type for your engine, gapped to spec — or clean and re-gap the old one in a pinch. A new plug is $6–12 and removes the doubt.

  1. With the plug wire off (see the gate above), remove the old plug and buy an exact match — the code is printed on the ceramic, or look up your engine model.
  2. Check the new plug's gap against the manual with a gap gauge; adjust gently on the side electrode only.
  3. Thread it in by hand until snug — small engines have soft aluminum heads, so no gorilla torque — then a firm quarter-turn with the wrench. Press the boot back on until it clicks.
  4. If a brand-new plug fouls again within a few uses, something upstream (rich carburetor, old fuel, choke stuck on) is the cause to chase next.
schedule15 min savings$6–15
Lawn mower spark plugMatch the code printed on the old plug's ceramic, or look up your engine model number

visibilityHow to check

  1. Turn the key. A rapid clicking, a slow groan, or dead silence — while the interlocks are all satisfied — points to a flat battery.
  2. If your model also has a recoil pull cord, try it. If the engine starts on the cord, the engine is healthy and the battery is the whole story.
  3. Put the battery on a charger overnight (many mowers charge through a plug-in port — check the manual). A battery that won't hold a charge after a full night is due for replacement.
buildNo tools schedule5 min

handymanHow to fix it

Charge the battery overnight with the charger your model uses; replace it if it won't hold a charge. Next winter, store the battery indoors or on a maintainer.

  1. Charge overnight using the method in your manual — many mowers have a plug-in charge port; others need the battery on a small automotive-style charger.
  2. If it cranks strongly after charging, you're done. If it's flat again next week, the battery is worn out — match the replacement's voltage and case size to the old one.
  3. Before next winter: store the battery somewhere heated, or leave it on a battery maintainer. Cold storage is what kills them.
schedule15 min savings$0–80

visibilityHow to check

  1. Do this outdoors, engine cold. Remove the air-filter cover and filter to expose the intake.
  2. Give one short burst of starter fluid into the intake, refit the cover loosely, and try to start it.
  3. Fires for a second or two, then dies: spark is fine and fuel isn't arriving — the carburetor is the prime suspect, especially after stale gas. No fire at all sends you back to the spark plug.
buildstarter fluid schedule10 min

handymanHow to fix it

report Fuel, springs, and tiny partsCarburetor work means open fuel — outdoors only, engine cold, plug wire off, nothing burning nearby. It also means small springs and gaskets that vanish into the lawn and an order of reassembly that matters. If any of that feels fiddly, stop guilt-free: a small-engine shop does this all day, and a seasonal tune with a carb clean typically runs $80–180 and comes back running like new.

Clean the carburetor: drain the bowl, spray the jets and passages with carb cleaner, and reassemble — or hand the mower to a small-engine shop for a spring tune that includes exactly this.

  1. Outdoors, engine cold, plug wire off, fuel shut off or the tank drained. Put a rag under the carb — some gas will escape.
  2. Remove the float bowl (usually one bolt underneath). Photograph everything before it comes apart.
  3. Spray carb cleaner through the main jet (often inside that bowl bolt) and the visible passages until they run clear. A single strand of wire clears a stubborn jet; never drill one.
  4. Reassemble with the gasket seated, refill with fresh fuel, and start it. If it runs then surges or dies, the deeper passages are still varnished — that's the point where the shop earns its fee.
schedule60 min savings$8–25
Carburetor cleaner sprayAerosol carb/choke cleaner with the straw nozzle

verifiedWhy this one goes to a pro

Diagnosing lost compression means opening the engine — valves, rings, head gasket — with specialty tools and torque specs. That's a small-engine shop's work. Get the diagnosis and quote first: on many walk-behind mowers the honest answer is that a repair costs close to a new machine, and a good shop will tell you so.

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

sort

Why the list is in this order

We don’t sort by likelihood alone. We sort the way a good, honest tradesperson thinks: rule out the free, safe, common causes first. That’s why the cheap checks outrank the expensive repairs — and why anything dangerous gets a hard stop instead of a pep talk. The order exists to save you money, not to sell you anything.