Why Your Lawn Mower Keeps Cutting Out

A mower that dies halfway across the lawn is almost always telling you one of three things: it is starving for fuel or air, it is being asked to cut more grass than it can clear, or something electrical is cutting the power. Petrol mowers usually stall from fuel and airflow problems, while cordless and mains models stop because a safety cutout has tripped. Work through the causes in order of likelihood and most stalls turn out to be a ten minute fix.

Start with the deck, not the engine

Before you touch a spark plug or a fuse, tip the mower up (fuel cap side up on a petrol machine, battery out on a cordless one) and look underneath. A deck packed with wet clippings is the single most common cause of a mower cutting out. The compacted grass jams against the blade, the blade slows, and either the engine bogs down and dies or the motor's overload protection shuts things off to save itself.

Scrape the underside clean with a plastic scraper or a piece of wood. Check the discharge chute and the grass box inlet too. If the box is full or the mesh is clogged with dried clippings, air cannot escape, clippings back up into the deck, and you get the same stalling behaviour even on a clean cut.

Cutting height and forward speed

If the deck keeps refilling, the mower is being overloaded. Raise the cutting height a notch or two and walk slower. Taking a third of the grass length at a time is the rule that keeps both the lawn and the mower happy. Long, wet or seedy grass may need two passes at different heights rather than one aggressive cut.

Petrol mowers: fuel, air and spark

Once the deck is ruled out, a petrol mower that keeps stalling nearly always has a fuel or air supply issue.

• Stale fuel. Petrol goes off surprisingly fast, and modern E10 fuel degrades quicker than the older E5 blend. Anything left in the tank over winter is suspect. Drain it, dispose of it properly, and refill with fresh fuel.
• Blocked fuel cap vent. This one catches people out constantly. The cap has a tiny breather hole. If it clogs, a vacuum builds in the tank, fuel flow stops, and the engine dies after a few minutes then restarts once the pressure equalises. Loosen the cap and run the mower briefly. If it no longer stalls, clean or replace the cap.
• Dirty air filter. A foam or paper filter matted with dust chokes the engine under load. Foam filters can be washed in warm soapy water, dried fully and lightly oiled. Paper filters get replaced, not cleaned.
• Fouled spark plug. Pull the plug and look at the electrode. Black and sooty means a rich mixture or a choked air filter. Oily means it may have been tipped the wrong way. Clean it up with a wire brush, check the gap against the manual, and fit a new one if the electrode is worn round.
• Carburettor gumming. If the mower runs for a minute then dies and fuel and air are both fine, old fuel has probably left deposits in the carb jets. Carb cleaner spray sometimes clears it, but a strip and clean is the reliable fix.

Check the oil level too. Many petrol mowers have a low oil sensor that cuts the engine to prevent damage, and overfilling causes its own problems, so aim for the mark rather than the maximum.

Cordless and electric mowers: cutouts, not faults

A battery mower that stops abruptly is usually doing it on purpose. Overload protection trips when the motor draws too much current, and thermal protection trips when the motor or the battery pack gets too hot. Both will stop the mower and both reset once things cool down.

If it restarts after five or ten minutes, that is your answer. Raise the cutting height, slow down, clear the deck, and let the pack cool between long sessions on a hot day. Repeated cutouts on easy grass point to a tired battery instead, especially if runtime has dropped noticeably.

On mains electric mowers, check the cable and the RCD plug. A nicked cable, a loose connection at the handle, or a tripping RCD will all present as a mower that keeps cutting out. Inspect the full length of the lead with the plug pulled.

Safety switches and handle levers

Every mower has a dead man's handle that kills the machine when released. Over time the cable stretches, the lever wears, or the microswitch gets dirty, and a slight change in grip is enough to break the circuit. If your mower cuts out when you turn or when the cable moves a certain way, look here. It is a cheap part and an easy replacement on most models.

The blade itself

A blunt blade tears grass rather than slicing it, which loads the motor far harder than a sharp one, and it leaves a ragged brownish finish. If your mower stalls more than it did last season and you have not sharpened the blade, that is a strong hint. Check the bolt is tight while you are down there, since a loose or bent blade puts strain on the bearings.