Garden power
Lawn Mower Won't Start? Troubleshooting Guide Before You Book a Repair
A petrol lawn mower that won't start almost always has one of five simple causes, and most of them you can check in under ten minutes without tools. Stale fuel, a dirty spark plug and a blocked air filter account for the great majority of no start faults. This guide takes you through each cause in the order to check them, and tells you where to stop and book.
In short: If your lawn mower won't start, check the fuel first: petrol goes stale in about 30 days and E10 in particular causes gum deposits that block the carburettor. Fresh fuel, a clean spark plug and a clear air filter fix most petrol lawn mower starting faults. Low oil and a flooded engine are the next things to look at before calling an engineer.
Before you start: remove the spark plug lead before touching the blade, cleaning the underside of the deck or working underneath the mower. Pull the rubber boot straight off the plug. For cordless mowers, remove the battery pack first. Work on level ground, let the engine cool, and keep children clear.
Lawn mower won't start and pulls freely
The cord pulls smoothly, you hear no attempt to fire, and the engine stays silent. The mower may have been stored over winter.
Stale fuel is the most common cause by a wide margin. Petrol starts to degrade in around 30 days, and E10 fuel draws in moisture as it sits, leaving a varnish like deposit in the carburettor jet that blocks the fuel feed completely. A dirty or fouled spark plug is the second most likely cause: carbon on the electrode stops it producing a strong enough spark.
Quick check: smell the tank opening. Stale fuel smells sour. If it has been in the mower more than a month, drain and refill with fresh petrol before anything else.
Fires once or twice then cuts out
The engine catches for a second or runs briefly, then dies.
A partially blocked carburettor or a dirty air filter is the usual cause. The engine can draw enough fuel or air to fire but not enough to sustain running. A clogged air filter starves the engine of air; a gummed carburettor jet delivers a weak fuel supply.
Quick check: open the air filter cover on the side of the engine. A foam filter should be springy; a paper one pale and clean. Black, oil soaked or collapsed means replace it.
Cord pulls hard or the engine feels seized
The starter cord resists strongly, or will barely move at all. There is no normal compression give.
Low or no oil is the most likely cause if this came on gradually: the engine runs low, overheats and seizes. Some mowers have an oil alert that locks the engine when oil drops below a safe level, so check the dipstick before assuming the worst.
Quick check: remove the dipstick, wipe and reinsert without screwing in, then read the level. Below minimum, top up with the correct grade and try again. Still immovable after that, stop and book.
Strong smell of petrol and the engine will not fire
There is a strong petrol smell and the engine will not catch or fires weakly for a moment.
The engine is flooded: too much fuel in the cylinder from over priming, a stuck choke or a leaking needle valve. The mixture is too rich to ignite. Flooding also happens when a mower is tipped on its side for blade checks.
Quick check: move the choke to run, hold the throttle fully open and pull firmly ten times. This clears excess fuel. If still silent, remove and dry the plug before trying again.
Starts fine but dies under load
The mower starts at idle but stalls as soon as it meets resistance.
This is not a starting fault. A partially blocked carburettor or a bent, dragging blade can cause this. The carburettor delivers enough fuel to idle but not enough under load: stop and book rather than adjust the carburettor yourself.
Quick check: with the plug lead off, look underneath at the blade. A bent blade drags on the grass: stop here and book.
Found yours? The fix is below. Rather not handle the spark plug yourself? Book a repair and we will do it.
How to check and clean a lawn mower spark plug
Checking the spark plug is the most effective first step when a petrol lawn mower won't start, and takes about five minutes.
- Remove the lead. Pull the rubber boot off the spark plug. Do this before anything else.
- Find the plug. It sits on the side or front of the engine. Clear any debris from around it before removing so nothing falls into the cylinder.
- Unscrew and inspect. Use a spark plug socket or a deep spanner to turn it anticlockwise. A normal plug electrode is tan or light grey. Black and sooty means a rich mixture or dirty filter; wet means flooding; white or blistered means overheating.
- Clean or replace. Clean light deposits with a wire brush. A cracked plug, a worn electrode or one that will not dry should be replaced. Check the part number on the old plug or the mower manual.
- Gap and refit. The gap should match your manual, typically 0.75mm to 0.8mm. Screw in by hand first, then tighten firmly.
- Reconnect and start. Push the boot back on. Try a cold start: choke on for the first two pulls, then off. If it still will not fire on a fresh plug, the fault is elsewhere.
How often to service a petrol lawn mower
Use fresh fuel and check your mower's E10 compatibility. Petrol degrades in around 30 days, and the government's E10 guidance advises checking whether your mower is rated for E10 before using standard pump petrol, as ethanol draws in moisture during storage. If in doubt, use super unleaded E5 or drain the tank before winter.
Lawn mower faults that are not the spark plug
If a fresh plug makes no difference, the fault is elsewhere, and repairing rather than replacing is nearly always the right call, as Recycle Your Electricals points out.
- CarburettorGum deposits from stale fuel block the jet: starts then dies despite a good plug.
- Air filterBlocked with grass dust: engine runs rich, loses power or will not sustain idle.
- Kill switch or cableA frayed or mis routed stop cable holds the engine off even with the handle released.
- Blade brake clutchOn blade brake clutch models, a worn pad kills the engine under load.
- Seized engineCaused by running without oil: needs an engineer and may not be economic to repair.
Worth repairing
A carburettor clean, new spark plug and fresh air filter cost a fraction of a new machine, and spare parts for most common engines are widely stocked. Nothing goes ahead without your say so: you approve any parts before we fit them.
Lawn mower starting questions
What are the most common reasons a petrol lawn mower won't start?
Stale fuel is the most common, followed by a dirty spark plug, a blocked air filter, low oil and a flooded engine. Most are things you can check yourself. If a lawn mower won't start after fresh fuel and a new plug, the carburettor is the likely next step.
How do I know if my lawn mower fuel is stale?
Stale petrol smells sour rather than sharp and may look discoloured or have a varnish like residue around the cap or fuel line. If the fuel has been in the tank for more than 30 days, especially over winter, treat it as stale: drain it and refill with fresh petrol before trying anything else.
Can I use E10 petrol in my lawn mower?
Check your mower manual. Many petrol mowers, especially older ones, are not rated for E10 and the higher ethanol content causes corrosion and gum deposits during storage. If in doubt, use super unleaded E5 or an alkylate petrol such as Aspen.
What should I do if my mower still won't start after checking everything?
Stop pulling the cord: repeated attempts flood the engine. If fresh fuel, a clean plug and a clear air filter have not fixed it, book a repair.
Is it worth repairing a petrol lawn mower that won't start?
In most cases yes. A carburettor clean, new spark plug and fresh air filter are modest costs. The decision changes if the engine has seized from running without oil, which can make repair uneconomical.