How To Tell If A Lawn Mower Spark Plug is Bad

How To Tell If A Lawnmower Spark Plug Is Bad + How To Replace Spark Plug

From this guide, you will learn How to tell if a lawn mower spark plug is bad. Because it’s a well-known fact – when your gasoline-powered lawn mower won’t start, the most likely cause is a bad spark plug. 

This guide will quickly help you find and remove your spark plug and use five (5) simple steps to determine whether your lawn mower spark plug is bad.

How to Tell If Your Spark Plug Is Bad

The most common way to tell if your lawnmower spark plug is failing is that the engine is more difficult 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 plug that’s the problem.

Your push mower engine may stutter and take several pulls to start. If you have a sit-on tractor mower, the engine may not start on the first turn of the key. When running, the engine may cut out, causing you 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 point to a bad lawn mower spark plug.

Best Way to Check Spark on Spark Plugs:

  • Step One:  Disconnect the Spark Plug Wire
  • Step Two: Remove the Spark Plug
  • Step Three: Inspect the Electrode on the Spark Plug for physical damage. Replace the spark plug if the electrode is worn or damaged or bad conditions exist.
  • Step Four:  Determine the Electrode Condition: Is it Dry, or is it Wet, gas, or Carbon fouled?
  • Step Five:  If the electrode is dry and shows no damage, Test the Spark of the spark plug, by doing a “test fire” of the spark plug. Watch this video to see How to Check Spark of Spark Plug :
  • Step Six:  If the Spark test produces a Spark, but not between the electrodes, the spark plug gap most likely needs to be set correctly.  
  • Step Seven:  If after adjusting the Spark Plug gap, the engine doesn’t fire between the electrodes, or doesn’t start you probably need a new spark plug.  We recommend changing a small engine spark plug once a year.

Note:  If you replace the spark plug and the engine won’t start, use this article to troubleshoot engine fuel (gasoline) problems

What does a bad spark plug look like?

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

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.

Q:  What Does a Spark Plug in a Lawn Mower Do?

A:  The spark plug is an electrical component that fits into 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.

Knowing why a well-conditioned spark plug is so essential to run a Lawnmower engine effectively is helpful to know the job of a spark plug. The spark plug’s job is the same as any other small engine.

Q:  Where does a Spark Plug get Power From?

A:  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 and side electrodes.

No current can flow because the fuel and air in the gap are insulators, 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, allowing the electrons to flow across the gap.  Small Engine – Spark plugs usually require 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 essential 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 identifying a lousy spark plug works equally well when inspecting spark plug condition for 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. Knowing the job of a spark plug may make it easier to understand what is happening.

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 becoming 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 –

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

Excess fuel consumption is another tell-tale sign that your lawn mower spark plug is bad. You may have noticed you need to fill the engine with gasoline more often than usual. Faulty spark plugs can often cause this.

If the gasoline isn’t wholly burnt off or inefficiently, 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: “The Top 6 Reasons Your Lawn Mower Won’t Start”. 

Frequently Asked Questions

Q:  How often do you change the spark plug in a Lawn Mower?

A:  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 is how well your lawnmower works and whether you have any issues.

In any case, I’d probably look to change your lawn mower spark plug a maximum of every two years 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.

Q:  How Do You Clean a Spark Plug?

cleaning spark plugs - best home gear
How To clean spark plugs –

A:  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 worn or rounded, you should change it.  Look for any cracks, chips, or physical damage to the porcelain sheath on the plug and any pitting on its firing electrode.

If there’s any damage or you’re unsure, 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:

Ways to Clean a Spark Plug:

A:  To Clean a Spark Plug: First, spray the spark plug with brake cleaner or starter fluid, and let it soak in for around ten minutes. Then wipe it with a clean cloth or rag to remove any excess residue. You can repeat this step 2 or 3 times if needed.

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

Q:  Can Using the Wrong Spark Plug Damage Your Engine?

A:  The short answer is yes. You need to use the specific spark plug needed for your mower, as different engine 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.

The wrong spark plug left in the engine for an extended period can cause problems with the piston rings.

Q:  What causes spark plugs to Fail?

A:  There are various reasons for spark plugs failing, but carbon buildup is common. A cracked distributor case, a clogged air filter, a dirty fuel injector, or an improper fuel-to-air mixture can cause this.

Another reason spark plugs fail is due to oil contamination. If oil seeps into the spark plug, it will damage the tip over time. If the seals 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.

Q:  How do you change a lawn mower spark plug?

A:  Replacing your lawnmower spark plug is straightforward and a more manageable lawn-mower maintenance task. You’ll only need a ratcheting socket driver, a spark plug socket, and a spark plug gauge tool.

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.

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. Next, follow the steps below to replace a spark plug:

  • 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 leaving 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. Always turn the spark plug by hand initially; until it doesn’t turn anymore, make sure you don’t cross-thread it.
  • Then use the spark plug wrench, a deep socket wrench, or a socket driver to tighten the plug. Don’t over-tighten. Make it snug; then, you need to connect the spark plug wire back securely on the top of the plug.

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

Q:  What size wrench do you need for a spark plug?

A:  You’ll need a socket wrench to remove a spark plug. They have rubber inside to protect the spark plugs’ ceramic insulator. Sparkplug sockets 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.

Q:  What does a white spark plug indicate?

A:  If your spark plug is white 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.

Q:  What does a Gap for a Spark Plug mean?

A:  For the spark plug to fire effectively, there must be the correct distance between the center and side electrodes of the spark plug.

It is called gapping your spark plugs when you adjust the distance between the two electrodes. 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.

Q:  What is my lawn mower spark plug gap?

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

Q:  Where can I buy replacement lawn mower spark plugs?

A:  Once you’ve got the model number from your spark plug or have referred to your user manual, you can search online for a replacement.

Amazon usually stocks 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 can find a suitable replacement, assuming they 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 MowerOr go with a Non-Motorized lawnmower with our Best Reel Mower Reviews

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

A:  In theory, yes, but it isn’t very specific in the case of a lawnmower. There’s less than a 1% chance of this ever happening, even if it leaks 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.

Q:  How to Tell if a Weed Eater Spark Plug is Bad?

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

Q:  How To Test Spark Plug With a Multimeter?

A:  Using a Multimeter also makes checking a Good vs. Bad Spark Plug easy.  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. 

Reading 5,000 – 15,000 Ohms is typical for small engine spark plugs.  The spark plug is bad if you get a reading outside that range.

Video – How to use a multimeter to check a spark plug


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References and Further Reading

Kevin Carroll
Kevin Carroll
Kevin is the author and editor for Best Home Gear, and uses his 25+ years experience in Commercial and Residential Construction Management to author and publish the work for this website. In addition to publishing Best Home Gear; Kevin enjoys the outdoors in Michigan and Arizona, hiking, cycling, fishing, golf, and completing DIY projects at his Home and Garden.


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Author and Publisher for Best Home Gear

Kevin is the Author, and Publisher @ Best Home Gear, which he began In 2018.

As a Professional in Construction, Real Estate and Property Management, Kevin uses that experience to publish Useful Articles, and help homeowners improve (DIY) their own homes, lawns, and outdoor spaces.

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