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How to Read a Spark Plug | Spark Plug Photos

Quickest Way To Inspect Condition of Spark Plugs and 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, along with our spark plug photos – will help you learn how to Identify the condition of any spark plug – and whether it’s worth saving or should be replaced. 


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 – Besthomegear.com

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.

Having a bad spark plug or fouled spark plug can also cause all sorts of problems in your Car, Truck, Lawn Mower, Chainsaw, or any other gas-powered engine. Symptoms can include; not being able to start your motor, engine misfires, poor gas mileage, and just overall bad performance.

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

Why Check your Spark Plug first?  

1) A bad spark plug is quite often the main 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 is likely to be a bad spark plug.

Routine Engine Maintenance:

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

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 easily inspect spark plugs for the condition:

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

Step 2:  Inspect Condition of Spark Plugs:

What do bad spark plugs look like?  After you remove a spark plug and inspect it carefully, 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 make sure the spark plug gap is consistent with the engine specifications for your engine.  

Simply insert the electrode over the Gap tool and slide until snug, this will determine the gap of the spark plug 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 not correct, the spark delivered will not match the spark intended for cylinder combustions for your engine, resulting in poor performance.


Determine The 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 – Besthomegear.com

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 brand of spark plug.


Wet Spark Plug (image)

Wet spark plug photo - Best Home Gear
Wet Spark Plug – Besthomegear.com

When lawn equipment won’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 this is the case, the wet spark plug can’t deliver spark to your engine. You can remove the plug and 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 – Besthomegear.com

If you see melted electrodes, blisters on the insulator, or any white deposits it is a sign of a burnt spark plug.  It is simply running way too hot for its application, and in outdoor lawn equipment, this can be a result of 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 – Besthomegear.com

If you discover oily deposits that are black on the insulator thread or the electrode it usually indicates oil is leaking past the pistons and into the cylinder of the engine.  If you are very mechanically inclined you can try and pinpoint the problem, or take it to a qualified small engine mechanic to diagnose.

If it’s a major repair you should weigh out 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 – Besthomegear.com

If you find a spark plug that looks 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 – Besthomegear.com

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 – Besthomegear.com


Congratulations – If you see brownish or gray deposits on the electrode like in this photo, and your electrode isn’t worn and the ground armature isn’t broken, 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, move ahead and check your gas fuel filter and air filter*. Make sure they are clean and look normal and replace them as needed. These last two repair checks can have a big impact on your engine performance.

Video from Motorweek:  “How to Read Spark Plugs”



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 just 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 other multi-cylinder engines, we highly recommend inspecting and replacing one spark plug at a time. This will help you avoid the mistake of 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 just as it was 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.

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

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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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