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How To Tell If A Lawn Mower Spark Plug is Bad

When you’re having problems with your gasoline-powered lawn mower, the most likely cause could be a bad spark plug.  Once you’ve determined where the spark plug is located, there are 5 simple steps needed to determine whether your lawn mower spark plug is bad.


How To Tell If a Lawnmower Spark plug Is Bad


Determine the Condition of Lawnmower Spark Plug using These 5 Steps:

  1. Disconnect the spark plug wire
  2. Remove the spark plug
  3. Inspect the Electrode on the spark plug for damage
  4. Determine the Condition of Electrode: Normal or is it Wet, gas or Carbon fouled
  5. Replace the spark plug if the electrode is worn or damaged or bad conditions exist.

The most common way to tell if your lawnmower spark plug is bad, is the engine being more difficult than normal to start.  Since the engine only starts due to two principles: Gas & Spark, and assuming you have filled the gas tank – it’s most likely the spark, or a lack thereof.

Your push mower engine may stutter and take several pulls of the cord to start. If you have a sit-on tractor mower the engine may not start on the first turn of the key. When it’s running the engine may cut out, causing you to have to attempt to restart your lawn mower’s engine again.

You may also experience loss of power and slower acceleration. These may appear subtle at the start but over time they can become more and more apparent, and these signs definitely point to a bad lawn mower spark plug.

What does a bad spark plug look like?

bad lawnmower spark plug - best home gear
inspecting Lawnmower Spark Plug – Besthomegear.com


To quickly determine the condition of Your Spark Plug: Check out This Photo Chart – which illustrates all 7 Spark Plug Conditions; then compare the photos to your own Spark Plug.

What Does a Spark Plug in a Lawn Mower Do?

To know why a well-conditioned spark plug is so important to run a Lawnmower engine effectively it’s helpful to know the job of a spark plug. The spark plug’s job is basically the same job as it would in any other small engine.

A spark plug is an electrical gadget that fits in the engine cylinder head, and its job is to spark the ignition of the fuel to create combustion within your engine. If it’s damaged or in bad condition, you may experience all sorts of issues.

Where does a Spark Plug get Power From?

The spark plug is connected to high voltage generated by an ignition coil or magneto. As the electrons flow from the coil, a voltage develops between the center electrode and the side electrode.

No current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes.

Once the voltage exceeds the dielectric strength of the gases, the gases become ionized. The ionized gas becomes a conductor and allows the electrons to flow across the gap.  Small Engine – Spark plugs usually require a voltage between 5-15,000 volts to ‘fire’ properly.

Other Signs That Your Spark Plug is Bad

We touched on the spark plug’s job above and it’s important to know this to help us understand why we may be experiencing some of the common symptoms of a bad spark plug.

We may have thought our lawnmower was experiencing engine problems and it could cost you hundreds or require buying a new mower, but it could simply be down to a damaged or faulty spark plug.

By the way, this information on how to identify a bad spark plug works equally well when inspecting spark plug condition for your any Gas-powered weed eater or Gas powered brush cutter.

Listen to Your Spark plug

As we mentioned once the lawnmower engine is running, it may cut out unexpectedly and for what appears to be no reason. Now we know the job of a spark plug it may be easier to understand what is happening.

The reason for the engine cutting out is often due to the spark being created from the spark plug being too weak to ignite the fuel which mixes with air in the carburetor. The result can often lead to the engine become flooded as more fuel enters the cylinder, due to it not being burnt off effectively.

Check Spark Plug Gap

Checking Spark Plug Gap - Best Home Gear
Check Lawnmower Spark Plug Gap – Besthomegear.com

As the engine has been running for long periods – the spark plug becomes hotter, which can increase the gap between the spark plug center and the firing electrodes, further weakening the ignition spark. This can be why you may need to leave the lawnmower to cool down if you’ve been trying to start it for a while but it keeps cutting out.

Excessive Gas Consumption

Some other tell-tale signs that your lawn mower spark plug is bad are excessive fuel consumption. You may have noticed you need to fill the engine with gasoline more often than usual. This can often be caused by faulty spark plugs.

If the gasoline isn’t being completely burnt off or inefficiently so your fuel consumption will increase and your fuel efficiency will plummet.

As the gasoline isn’t being burned off as it should you may also notice the smell of fuel in the air – often it’ll smell like raw gasoline.

Video: Let’s check out”The Top 6 Reasons Your Lawn Mower Won’t Start”. 



Frequently Asked Questions


How Often do you change the spark plug in a Lawn Mower?

How often do I need to change my lawn mower spark plug? Most experts claim you should replace the lawnmower spark plug in a standard mower once per season, or after around 25 hours of use.

I believe that may be overly cautious and you may get a little more use out of it before you have to change the spark plug.  A good gauge would be how well your lawnmower is working and whether you’re having any issues.

In any case, I’d probably look to change your lawn mower spark plug a maximum of every two years just to be safe, even if it’s still performing well. They’re very cheap, and you can usually pick them up for only a few dollars.

How to Clean Spark Plug:

cleaning spark plugs - best home gear
How To clean spark plugs – Besthomegear.com

The age-old question: Can you clean spark plugs and if so, How do you clean spark plugs?  That’s easy – Yes you can clean a spark plug – assuming it is just wet (too much gas or oil build-up) and not corroded too badly (from carbon or burnt deposits).

When you remove the spark plug from a lawnmower – it will immediately give you clues as to whether it needs to be replaced or whether it can just be cleaned up.

The center electrode (the part that delivers the spark) should have a flat even surface on top. If you find it is worn or rounded then you should change it.  Look for any cracks, chips, or physical damage to the porcelain sheath on the plug, as well as any pitting on its firing electrode.

If there’s any damage at all or you’re unsure then it’s best to replace it, as they’re very inexpensive.

If the plug is in decent condition, but has some black discoloring due to oil, or is wet with gasoline then this can usually be cleaned up.

Cleaning Lawnmower Spark Plug:

To Clean, the Spark Plug – spray it with some brake cleaner or starter fluid, and let it soak in for around 10 or so minutes. Then wipe it with a clean cloth or rag to make sure any excess residue is removed. You can repeat this step 2 or 3 times if needed.

Once the spark plug is clean you should inspect the plug again, determine if the spark plug threads look good – with no cross-threading evident, and the electrode looks good.   If you notice at this stage there’s some previously unseen damage then it’s best to replace it.

Can the wrong spark plug damage your engine?

The short answer is yes! It’s important you research which spark plug is needed for your mower as different models require different plugs.

A common sign the wrong spark plugs have been installed is if the electrodes are flattened or broken off. A spark plug that is too long can cause extensive damage to your lawn mower’s engine, whereas a plug that is too short can cause poor performance and fuel efficiency.

A spark plug left is left in the engine for an extended period of time can cause problems with the piston rings.

What causes spark plugs to fail?

There are various reasons for spark plugs failing but a common one is carbon buildup. This can be caused by a cracked distributor case, clogged air filter, a dirty fuel injector, or improper fuel to air mixture. Another reason they fail is due to oil contamination.

If oil seeps onto the spark plug, it will damage the tip over time. If the seals begin to fail then oil can leak into the combustion chamber.

Once You’ve determined the Spark Plug needs Replacing – Refer to the instructions below:  How To Change a Lawn Mower Spark Plug.

How do you change a lawn mower spark plug?

Replacing your lawnmower spark plug is pretty straightforward and one of the easier lawn mower maintenance tasks. The only tools you’re likely to need are a ratcheting socket driver, a spark plug socket, and a gauge. Make sure the engine is cool before you attempt to replace the spark plug.

For the novice who may be asking where is the spark plug on a lawnmower, the spark plug can be found beneath the black spark plug wire. Simply, unsnap (disconnect) the spark plug wire and you can now remove and inspect the lawnmower spark plug.

  • The first thing you should do is check your replacement plug is the same model and shape/size as the one that’s been replaced. You can usually match them using the reference numbers on the white insulator.
  • Disconnect the spark plug lead and clean around it
  • Remove the old spark plug using a socket driver or wrench. If you’re having trouble removing the old spark plug you can try adding some lubricant and leave it to soak for 10 minutes.
  • Once the old spark plug is removed inspect the electrode to see how the engine is running. If it’s too wet there may be a problem with the engine choke system. If it’s powdery and seems too dry there could be a problem with the carburetor mounting gasket or it could be starved of oil.
  • The final step is to insert the new spark plug into the hole of the engine. Remember to always turn the spark plug by hand initially, and until it won’t turn anymore to make sure you don’t cross-thread it. Then use the spark plug wrench, a deep socket wrench or socket driver to tighten the plug. Don’t overtighten, just make it snug, then you just need to connect the spark plug wire back securely on the top of the plug.

Spark Plug Installation Video: “How to Check, Clean, And Replace a Lawn Mower Spark Plug”

What size wrench do you need for a spark plug?

You’ll need a socket wrench to remove a spark plug. They have rubber inside to protect the spark plugs ceramic exterior. They can come in sizes from 13/16 inches to ¾ inches.

Check the size of your spark plug and match the socket to that. They’re readily available online or in hardware stores.

What does a white spark plug indicate?

If your spark plug is white in color or looks to be blistered it means the plug is becoming too hot. This could be down to the engine’s cooling system or incorrect ignition timing.

What does a gap for a spark plug mean?

In order for the spark plug to fire effectively, there must be an exact distance between the center and side electrodes of the spark plug.

When you adjust the distance between the two electrodes it is called gapping your spark plugs. You’ll need a feeler gauge to gap your spark plug properly.

The gap must be a specific width if the gap is too small the spark doesn’t have enough space to generate the power needed to ignite the fuel.

What is my lawn mower spark plug gap?

You’ll need to go to the manufacturer’s website or read the manual to see the exact gap in your lawnmower spark plug. However, the gap usually ranges between .020” and .030”.

Where can I buy replacement lawn mower spark plugs?

Once you’ve got the model number from your spark plug or have referred to your user manual you can go about searching online for a replacement.

Amazon usually stocks pretty much all shapes and sizes so that could be your first port of call.  There’s also the option of going to your local hardware store, where you should be able to find a suitable replacement, assuming the stock a wide range of engine parts.

Need a recommendation on a Fantastic Cordless Electric Mower that does not require a Spark Plug?  Check out our favorite – the Greenworks 21″ 40V Brushless Cordless Lawn Mower

Or go with a Non-Motorized lawnmower with our Best Reel Mower Reviews

Can a faulty spark plug cause an engine to catch fire?

In theory yes but it’s very unlikely in the case of a lawnmower. There’s less than a 1% chance of this ever happening, even if it is leaking fuel.

The engine is much more likely to fail before it catches on fire. If there are any warning signs though or you’re worried it’s best to consult an engine specialist for your peace of mind.

How to Tell if a Weed Eater Spark plug is Bad?

Since lawn mowers and gas-powered weed eaters run on very similar spark plugs, the 5 steps to follow at the beginning of this article – would be the same steps for weed eaters (string trimmers).

How To Test Spark Plug With a Multimeter?

Using a Multimeter is also an easy way of checking a Good vs. Bad Spark Plug.  Simply Set the Multimeter to “Ohms”, Then test by tapping the two leads together to show Zero Reading – Next, connect one lead to the spark plug electrode and one to the top of the spark plug.  Any reading of 5,000 – 15,000 Ohms is in the normal range for small engine spark plugs.  If you get a reading outside that range, the spark plug is bad.

Check out this video – how to use a multimeter to check a spark plug



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References and Further Reading



  1. Would like to no if anyone can tell me what’s wrong with my mower it starts n runs but when full Throttle it starts to rev high n low it’s like it’s going to stop

    • Hi and thank you for posting a comment – I hope the article helps you quickly determine how to remove, inspect and change spark plugs in the future.

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About the Author: As a Builder, Realtor, and Property Manager for 25+ years, Kevin has used Tools and Outdoor Equipment for the bulk of his career. Managing property in Michigan and Arizona, he also tests many of the products he reviews firsthand, in vastly different climates and conditions. Our goal at BestHomeGear.Com is to share first-hand experience - Providing "Unbiased" Product Reviews, DIY Guides, and useful Home & Garden Techniques for our faithful readers - Homeowners just like you!

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