Hidden lawn mower issues: how to identify and resolve problems you didn't know you had

Our Which? gardening experts have listed seven lawn mower problems and how to fix them. Our advice is that maintaining a lawn mower is just as important as regularly servicing your car.

Some issues could lead to a mower breakdown, not just poor performance. So we’ve put this article together to try and help you identify the most common hidden lawn mower faults. 

In the market for a new mower? Check out our guide to the best lawn mowers for 2023.

Dull blades

Signs: Unevenly cut grass, takes a longer time to mow, lawn has a straggly appearance.

Resolution: Regularly check your mower blades for dullness. A sharp blade cuts grass cleanly, while a dull blade tears the grass, resulting in a ragged edge and an unattractive, patchy appearance. 

If the blade is dull or damaged, either sharpen it using an angle grinder or replace it entirely. Be sure to take extreme care – you’ll want protective eyewear, earplugs and gloves. 

You also have the option of taking the blade to a repair or hardware shop.

If your mower can't fixed, check out out guide to how to buy the best lawn mower

Clogged deck

Signs: Decreased cutting efficiency, mower leaves clumps of grass on the lawn, hard work to push the mower, excessive vibration.

Resolution: Regularly clean the under-deck area of your mower. Grass clippings, dirt, and debris can accumulate under the deck, affecting the mower's cutting efficiency.

Ideally you should clean your lawn mower after each use, especially if you're mowing wet grass. If that's not possible, attempt to clean it once a month at the very least. Any debris stuck to the deck of your lawn mower can be scraped and sprayed away using a wire brush, a damp cloth or compressed air. 

Some petrol mowers even feature a 'washout port' – a hose is directly inserted into this to flush out debris. However, we wouldn't advise using your garden hose on any electric mowers since you run the risk of water getting in and damaging electronic components.

Faulty spark plug (petrol lawn mowers)

Signs: Difficulty starting the mower, the mower stalling frequently, or a noticeable decrease in engine power.

Resolution: Make sure the mower is turned off, then inspect the spark plug for signs of wear or damage. 

If the spark plug is visibly damaged, corroded, or covered in deposits, it's time to replace it. If it's just dirty, you can clean it and adjust the electrode gap using a spark plug gap tool.

Generally, you should replace your spark plug annually to keep your mower running as efficiently as possible.

Check out our guide to the best lawn mower brands to find out which manufacturers are producing the most durable models.

Stale fuel (petrol lawn mowers)

Signs: The mower won't start, or it starts then quickly dies.

Resolution: Fuel that has sat in the mower for an extended period – over winter, for example – can become stale and may prevent the mower from starting.

You’ll need to drain the fuel from your lawn mower and replace it with fresh petrol. Regular unleaded petrol will work in your petrol lawn mower, but it can be worth opting for a higher quality petrol with a lower ethanol content as it will absorb less moisture from the air.

At the end of each mowing season, it's best to run the mower until it uses all the fuel. Or, you can use a fuel stabiliser to prevent the fuel from degrading.

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Loose or worn belt

Signs: Less powerful drive, diminished blade speed, a squeaking sound when the mower is running.

Resolution: Regularly inspect the mower's drive belt with the engine switched off. If the belt is loose, it may need to be tightened. 

If it's cracked or worn, it should be replaced. Keep in mind that replacing a lawn mower belt can be complex, so it might be best to seek professional help if you're uncertain.

Get more expert tips in our guide on how to maintain your mower and cut the perfect lawn.

Worn-out wheels

Signs: Difficulty pushing the mower, uneven grass cutting, wobbly movement.

Resolution: Check your mower's wheels for wear and tear. Worn-out wheels can make the mower hard to manoeuvre and result in an uneven cut. If the wheels are worn or damaged, replace them.

Reduced battery life (cordless lawn mowers)

Signs: The mower's run time is significantly reduced, or the battery takes longer to charge.

Resolution: Like all batteries, the ones in your cordless mower have a limited lifespan and will eventually need to be replaced. If the battery isn't old but is still underperforming, ensure the mower and battery are kept in a clean, dry location when not in use. 

You should also check the battery connections, and clean them if needed. 

Extreme temperatures can also shorten a battery's lifespan, so consider this when storing. During the off-season, it's important to recharge your battery several times, even if you're not actively using it. Allowing the battery to deplete and remain at 0% charge can potentially lead to a loss of ability to hold any charge when you need to use it again.

We make sure to record the battery life of every cordless mower we test, so that you can be assured your machine has enough juice to get the job done. Read our cordless lawn mower reviews to find the best of the bunch.