How Long Does A Lawn Mower Spark Plug Last (When to Replace)

How Long Does A Lawn Mower Spark Plug LastIf you own a fuel powered lawnmower, this comes with added maintenance. One of the main components which is required to make a lawnmower run is a spark plug.

Like a car, this helps ignite the lawnmower engine and makes it run; but how long does a lawnmower spark plug last?

We will also look at the symptoms of a bad spark plug and whether you can clean and re-use one or not.

How long does a Lawnmower spark plug last?

There is really no end date to a spark plug, generally speaking, the spark plug in lawn mower will last at least around 25 hours or up to 2 years of use. However, a spark plug may not last that long due to the following factors and reasons which could cause a detrimental effect on the spark plug.

These factors include:

  • Improper use (i.e. High unnecessary revving of the engine)
  • Overusing the lawnmower in each session
  • Lack of general maintenance of lawnmower
  • Cutting wet grass (as it is harder to cut, the mower will use more power and consequently use the spark plug more)

lawn mower spark plug lifespan

What makes spark plugs go bad in lawn mower?

The most common reason why a spark plug goes bad is the carbon build up, this is caused by incomplete combustion, where not all the fuel is being ignited properly within the carburettor which will cause smoke and give out the carbon deposits which will cover the spark plug. Carbon build-up caused by incomplete combustion can be caused a few reasons: one of which is by an overly ‘rich mixture’, this means there is too much fuel in comparison to air. If there is not enough air to fuel ratio, there will be excess fuel that cannot combust which causes improper ignition of the fuel to cause the carbon deposits.

It is best to check and clean the air filter once you have used the lawnmower a good few sessions. If there is a lot of debris and dust in the filter this will block off the necessary air required. The air filter is usually located near the top of the engine which might be encased by a plastic or metal cover. Some lawnmowers may have their own and more sophisticated filter, it is best to check your owner’s manual to find where this located.

Another reason is if your lawnmower is using old fuel. As fuel gets older it will thicken and decompose, this will make it harder to ignite and will burn the fuel inefficiently which will produce smoke and cover the spark plug with carbon. So if the next time you come to use your lawnmower and it is likely to sit in the shed for over 3 months before your next cut, be sure to empty the tank and replace with new clean fuel when you come to use it. This will also help keep the carburetor in good condition and avoid it clogging up!

If you are using it on a cold day, it is best to leave your lawnmower to warm up before using it at full power to cut your grass. Using your lawnmower on a cold day without warming up doesn’t give the best conditions for the spark plug to work effectively and efficiently, which again will cause incomplete combustion. If there is not enough heat being produced from the spark plug, this will not ignite all the fuel effectively which causes carbon deposits.

On the side of the spectrum, overheating can also cause the spark plug to fail. If there is excessive use of the lawnmower or if the lawnmower is using more power than usual more heat will be produced and could coincidentally overheat the spark plug which will wear away it away. It is best to use the lawnmower in the optimal conditions it is meant for such as cutting dry grass and ensuring any general maintenance has been done on it. It is also ensuring the correct spark plug is being used with your specific engine too as all engines vary and have different heat requirements.

Oil contamination is an uncommon problem but can happen. As a spark plug is not designed to be used with oil, the oil will damage the spark plug tip which helps ignites the fuel to make the lawnmower run. It is best to check any oil seals to ensure they are not leaking onto the spark plug. If there is a leak, ensure to read through the owner’s manual to diagnose and fix the problem to avoid this problem from happening. By checking for any leaks this also inadvertently likely stop you having to purchase a new lawnmower as the oil helps keep all the moving components well lubricated and avoid any mechanical problems along the way!

How often should a lawn mower spark plug be changed

Generally, the lawnmower spark plug is recommended by manufacturers, to be changed once it has had 25 hours of use or at the start of the new season. This recommendation doesn’t mean the spark plug is not in working order although you may find various symptoms which indicate the spark plug should be changed which is further explained later on.

But of course, when the spark plug should be changed is also dependant on how well you maintain your lawnmower. Every so often you should carry out the necessary maintenance once in a while as specified in your owner’s manual (i.e. fuel changes and cleaning) which can help prolong the life of the spark plug. Additionally, you do have the choice to clean and maintain the spark plug (explained further later) but eventually, the spark plug will be required to change.

Having said all this, you should change your spark plug no matter what (despite if the lawnmower is still performing well) at least every 2 years at most as you may be damaging the engine without knowing.

Also read: When To STOP Mowing your Lawn in the Fall

How do I know if my lawn mower spark plug is bad?

There are various ways you could find out instantly if your spark plug is bad. There are essentially two main methods to tell.

One method is to see whether your lawn mower starts easily. If it takes several more times to tug on the cord than usual to start or doesn’t start at all that is a sign that it is the spark plug. Other symptoms can include excess fuel usage, stuttering of the engine while in use (various symptoms of this are explained later on). If the check has be done, just make sure the spark plug is in correct place and inserted properly and not loose as there may not actually be anything wrong with it!

Another method is a visual inspection of the actual spark plug, if it looks worn or coated in black that is a clear sign of a well-used spark plug that is not able to be used to its full potential, this can be cleaned and maintained and is explained later on.

Symptoms of Bad Lawn Mower Spark Plug

As previously mentioned the most obvious symptom is how easy the lawnmower is to start or not start at all. If it takes several more pulls than usual, it is a sign the spark plug isn’t working well in order to create the adequate spark to ignite the engine. Alternatively, whilst the engine is running and it suddenly starts stuttering or misfiring while in use this can be a sign of a bad spark plug. Other symptoms also include excess use of petrol (due to incomplete combustion), whereas previously mentioned is usually due to carbon build up or a worn out spark plug.

Visual inspections can be made, the easiest way to tell is the colour of the spark plug. Is it covered in black soot-like dirt? If yes, that is the obvious sign of a bad spark plug. This could be cleaned however, which is explained later on. Another sign is to look at the electrode (small bent piece of metal) of the spark plug, this should have a flat top if new, if used you can expect a bit of rounding. But if it quite rounded on top or if cracks are appearing, it is time to replace it. Should you find the spark plug oily, this may mean you have an oil leak and this will prevent your spark plug from working properly too and is a good time to find the leak and repair the mower as necessary.

There is also something called the ‘electrode gap’ (between the two electrode tip and bent metal). This gap essentially creates the spark to make the engine start, if there is no gap or a very small gap, this is a sign of the spark plug going bad.


What does a bad lawn mower spark plug look like

There are different ways a bad a plug could look:

Normal looking spark plug: Brown or grey like tan deposits on a spark plug and can be carried on using

normal looking lawn mower spark plug

Carbon deposited spark plug – As seen in the image, the electrode is covered in black, this is the carbon deposits. This can be cleaned and reused.

Carbon deposited spark plug

Oily deposited spark plug – Looks ‘cleaner’ but don’t let it fool you, it means there is an oil leak somewhere which needs to be addressed first before replacing the spark plug

Oily deposited spark plug

Burned spark plug (overheated) – White deposits or a melted electrode, spark plug needs replacing

Burned lawn mower spark plug


Worn or broken electrode – A rounded or broken electrode tip, the spark plug will need replacing

Worn or broken electrode on lawn mower spark plug

Can I clean and reuse lawn mower spark plugs & How to do it.

If the electrode is rounded and not flat or burnt, the spark plug is well worn and cannot be cleaned or reused, a replacement is required. However, if the electrode tip looks fairly flat with only some black carbon deposits this can be cleaned. Using a soft wire brush, carefully brush and rotate the spark plug to clean the black deposits off until you start seeing some of the silver as if it was like new!

If just using the brush isn’t cutting it, use a cloth with brake fluid or starter fluid, this will help break down the carbon and make it easier to clean. If you don’t have either of those, WD40 will also do the trick but may not be as effective.

Should you find other problems such as white deposits or cracks in the electrode which the carbon covered up, replace the spark plug.

Related Questions.

Can a lawn mower start without a spark plug?

If you have a fuel powered lawnmower, in short, no, the lawnmower cannot start. Even if it does start the engine will soon cut out as the spark plug is needed to create the ignition of the fuel to make the engine work.

If you have a battery-powered or electric lawnmower this doesn’t need a spark plug as it is all fully electric.

How much does a lawn mower spark plug cost

A spark plug is fairly cheap to replace and should be no more than $5-$7 which you should be able to get at your local hardware store or online. But be sure to read the owner’s manual to find the correct one specifically for your lawnmower.