How to Read a Spark Plug | Spark Plug Photos

How To Inspect Spark Plugs + How to set Spark Plug Gap

Are you wondering How to Read a Spark Plug?  If so, today, we can save you considerable time diagnosing (or misdiagnosing) your spark plug conditions and save gasoline engine performance. 

This article and our spark plug photos will help you identify any spark plug’s condition and whether it’s worth saving or should be replaced. 

How to Read a Spark Plug

how to read spark plug - best home gear
How To Read A Spark Plug –

Step1:  Learn The Parts of a Spark plug

Refer to our Spark Plug diagram below – Familiarize yourself with the name of each spark plug component so you can quickly inspect spark plugs for the condition:

parts of a spark plug - best home gear
Parts of A Spark plug –

Step 2:  Inspect the Condition of Spark Plugs:

What do bad spark plugs look like?  After you remove a spark plug and scrutinize it, you will likely see one of these 7 Spark Plug conditions:

  • Carbon fouled
  • Wet (gasoline)
  • Burnt
  • Oil deposits
  • Worn out Electrode
  • Broken Electrode
  • Normal

Step 3:  Look for Permanent Spark Plug Damage:

Once you remove and examine the spark plug, look for any signs of visible damage, like a broken or worn electrode, terminal damage, or crack in the spark plug insulator.

Step 4:  Check Spark Plug “Gap”:

Spark Plug Gap Tool - Best Home Gear
Gapper Spark Plug Tool

Use a Spark Plug Gauge like this one from Gapper to ensure the spark plug gap is consistent with the engine specifications for your engine.  

Insert the electrode over the Gap tool and slide until snug; this will determine the spark plug gap you are holding.

Use the Gap Opener portion of the gap tool, and open or close the side electrode (prong) – to match the gap specifications for your engine.

If the Spark Plug Gap is incorrect, the spark delivered will not match the spark intended for cylinder combustions for your engine, resulting in poor performance.

Diagnosing Spark Plug Condition:

As you examine it, determine what the condition of your spark plug is by matching your spark plug to the photos below:

Carbon Fouled Spark Plug (image)


Carbon Fouled Spark Plug –

If you see black deposits on the electrode or insulator of your spark plug, you have a carbon build-up – or carbon-fouled plug. Carbon-fouled plugs indicate your fuel-to-air carburetor settings are too rich in gas or your air filter is dirty and needs to be cleaned or replaced. 

You may also consider using a “hotter” or different recommended spark plug brand.


Wet Spark Plug (image)

Wet spark plug photo - Best Home Gear
Wet Spark Plug –

When lawn equipment doesn’t start, and you’ve pulled the start rope  4 or 5 (or 20) times, you may have flooded your engine and created a wet spark plug. If true, the wet spark plug can’t deliver a spark to your engine. You can remove the plug, dry it off, wait a few hours, or try it another day after it dries out.


Burnt Spark Plug (image)

burnt spark plug - best home gear
Burnt Spark Plugs –

It is a sign of a burnt spark plug if you see melted electrodes, blisters on the insulator, or any white deposits.  It is simply running way too hot for its application, and in outdoor lawn equipment, this can result from using the wrong spark plug, incorrect gas/air mixture, or a loose spark plug. Correct the problem, then replace the spark plug.

Oil Deposits Spark Plug (image)

Spark Plug with Oil Deposits - best home gear.
Spark Plug with Oil Deposits –

Suppose you discover black oily deposits on the insulator thread or the electrode. In that case, it usually indicates oil leaks past the pistons and into the engine’s cylinder.  If you are mechanically inclined, you can try and pinpoint the problem or take it to a qualified small engine mechanic to diagnose.

If it’s a significant repair, you should weigh the costs; it could be time for a new mower vs. fixing the old one.

Worn-out Electrode Spark Plug (image)

Spark Plug with worn electrode - best home gear.
Spark Plug with Worn Electrode –

If you find a spark plug similar to this photo, you’re looking at a spark plug with a worn-out center electrode.  Replace the plug, and you should be fine.

Broken Electrode Spark Plug (image)

spark plug with broken electrode - best home gear
Spark Plug with Broken Electrode –

If you or someone else installed the wrong size spark plug, and it was too long for your engine, you may find a flattened or broken side electrode.  This type of spark plug can cause significant engine damage, while installing a spark plug that is too short will cause poor gas usage.

Normal Spark Plug (image)

Normal Spark Plug
Normal Spark Plug –


Congratulations – If you see brownish or gray deposits on the electrode like in this photo, your electrode isn’t worn, the ground armature isn’t broken, and your spark plug is Normal. You can go ahead and check the spark plug gap and reinstall it.  

Recommendation:  If your spark plugs all check out Normal, check your gas fuel and air filter*. Make sure they are clean and normal, and replace them as needed. These last two repair checks can greatly impact your engine performance.

 What Your Spark Plugs Can Tell You:

Spark plugs are the all-important device that delivers the necessary Spark to your engine and, when combined with the proper gas-air mixture, provide the necessary combustion to run your motor. 

And a spark plug’s condition can provide tell-tale signs about the condition of your engine

how to read spark plug - best home gear
How To Read Spark Plugs –

By simply reading spark plugs for wear and deposits, you can quickly discover whether your spark plugs are either worn out, getting too much gas (rich), too little gas (lean), running too hot (burnt), wet, cracked, and whether it’s time to change them.

A bad or fouled spark plug can also cause problems in your Car, Truck, Lawn Mower, Chainsaw, or any other gas-powered engine. Symptoms include the inability to start your motor, engine misfires, poor gas mileage, and bad performance.

If you notice any of these performance issues with your vehicle or Lawn equipment, your spark plug is the best place to start.  

Why Check your Spark Plug first?  

1) A bad spark plug is often the leading cause of poor engine performance.
2) Spark plugs are very inexpensive to replace. It’s the cheapest place to start.
3) If you haven’t changed your spark plugs in 2 years or more, your problem will likely be a bad spark plug.

Routine Engine Maintenance:

Every vehicle owner, landscape professional, and homeowner must regularly check their lawn equipment’s performance. The good thing is that examining spark plugs allows you to “Instantly Read”  the condition of this vital piece of your engine’s operation.

Video from Motorweek:  “How to Read Spark Plugs”


Frequently Asked Questions:  (FAQ)

Q:  How Do I Know If Spark Plug Is Bad on Mower?

A:  The simplest way to determine if your lawnmower spark plug is bad is to remove and inspect its condition.  For instance, if the spark plug has a worn or broken electrode (prong), you must replace it.  Additionally, if the spark plug has oil deposits or is burnt, it should be replaced.

Sometimes, the spark plug needs to have the spark plug “Gap” set correctly.

When in doubt, we recommend replacing a spark plug if you suspect it is causing the engine to fail, as spark plugs are inexpensive.

Q:  Should You Replace Wet Spark Plug on A Mower?

A:   A wet spark plug on a lawnmower typically indicates it has been flooded with too much gasoline. Wet spark plug will happen when you pull the pull cord many times without starting the lawnmower. 

We recommend removing and inspecting the spark plug for damage or, if wet, dry it off and reinstall it.  Note:  Be sure you have “Fresh Gas” in your mower before starting it again, which is a leading cause of mowers failing to start.

Q:  How Often Should You Change a Lawnmower Spark Plug?

A:  To keep a Lawnmower engine running smoothly, the spark plug should be changed at least every two years.

Q:  How Do You Set Spark Plug Gap on Mower?

A: Knowing how to Use a Spark Plug Gauge is vital to ensuring the spark plug gap is consistent with the engine specifications for your lawnmower engine.  

Step 1:  Insert the electrode over the Gap tool and slide until snug; this will determine the spark plug gap you hold.

Step 2:  Use the Gap Opener on the gap tool to open or close the side electrode (prong) and match the gap specs for your small engine.

If the Spark Plug Gap is incorrect, the spark delivered will not match the spark intended for cylinder combustions for your engine, resulting in poor performance.


Determining the condition of spark plugs is simple once you know how to read a spark plug. And now that you know what to look for – you need to get familiar with the simple steps required to change a spark plug.

If you’re changing or inspecting multiple spark plugs on a vehicle, lawnmower, or multi-cylinder engine, we highly recommend inspecting and replacing one spark plug at a time. This will help you avoid re-connecting the wrong spark plug wire.

Check online or with your local hardware or auto parts store. Once you’ve located and replaced the correct model spark plug(s), gas, and air filters, your engine should perform as designed.

Note:  If you’re inspecting Spark plugs on Outdoor lawn equipment and aren’t very familiar with small engine maintenance – check out how to tell if a lawn mower spark plug is bad

Note: The contents of this article are designed for informational purposes and to help you quickly navigate the various conditions.  Under no circumstances should you avoid the advice of your local mechanic or small engine repair professional.

Please seek the advice of a certified technician if you have any questions or concerns related to the content of this article.

Further Reading

Need a New leaf blower? Top 5 Best Leaf Blowers – Cordless and Electric

How To Read a Spark Plug – Krockfd1, CC BY-SA 4.0, via Wikimedia Commons

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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