How to Read Your Spark Plugs?

Reading a spark plug consists of examining and evaluating the condition and color of the tip of the plug. Learn how to read a spark plug so you can know how your vehicle is performing, foresee potential problems and take care of them early, and have the knowledge of when to change your Spark plugs for best performance.

Why Do You Need to Read Spark Plugs?

Reading spark plugs means evaluating their condition. This is typically done by inspecting the spark plug tip’s coloration, and checking if the tip is damaged. An engine that’s functioning properly will have spark plugs with a brown or light gray color around the tip.

However, if the spark plug tips have a different color, then it can mean that your engine has a problem with its ignition or fuel system.

Different spark plug tip colors can correspond to specific problems with your engine. A black spark plug tip means oil is getting inside the combustion chamber, while a white spark plug tip is a sign that the spark plug is overheating.

While a spark plug tip’s color is a great indicator of problems, you should still be on the lookout for signs of corrosion, melting, or damaged electrodes.

 If your spark plug tips have signs of fouling or overheating, the problem must be quickly diagnosed to prevent further issues.

Spark Plug Reading Chart

Spark Plugs Reading Chart

How Do I “Read” A Spark Plug?

Being able to “read” a spark plug can be a valuable tuning aid. By examining the insulator firing nose color, an experienced engine tuner can determine a great deal about the engine’s overall operating condition.

The first step is removing the spark plug you want to look at. Follow these instructions on how to remove a spark plug. Take the spark plug in your hand and begin looking at it for signs of damage. Look at end of the spark plug that was in the cylinder and examine it. You will likely see one of the following conditions.

How to Read Your Spark Plugs

Normal Spark Plug

A normal spark plug will have brown or grayish-tan deposits on the side electrode. Everything is just fine with your spark plug; you can reinstall the spark plug.

Carbon Fouled

Black, dry soot on the electrodes and insulator tip indicates a carbon-fouled plug. This can be caused by a dirty air filter, excessive driving at low speeds, too rich of a fuel/air mixture or idling your vehicle for too long.

Your mechanic can offer you advice on what type of spark plug to buy to replace the damaged plug, but you may want to consider switching to  a “hotter” spark plug (the higher the spark plug number, the hotter the plug).

Oil Deposits

Black, oily deposits on the electrodes and insulator tip points to an oil-fouled plug. Oil could be leaking into the cylinders, getting past pistons that are worn or valve guides that are worn.

Finding the source of the leak is very important – consult a mechanic for guidance. Once the problem is addressed, you can replace the spark plug.


A wet spark plug can be the result of the engine flooding. Flooding is what happens when you try to start the engine several times without it firing up. You can clean the spark plugs or you can just wait for them to dry out.


Blisters on the insulator tip, melted electrodes, or white deposits are signs of a burned spark plug that is running too hot. Causes can include the engine overheating, incorrect spark plug heat range, a loose spark plug, incorrect ignition timing or too lean of an air/fuel mixture. The spark plug should be replaced.

Worn Electrodes

Worn and eroded electrodes are symptoms of a spark plug that is past its prime. The plug has been in the engine too long and needs to be replaced.

Broken Electrodes

If the electrodes are broken off or flattened, it is likely that the wrong spark plugs are installed.  A spark plug that is too long can cause extensive damage to your engine while a short spark plug can cause poor gas mileage and spark plug fouling. Check your owner’s manual to ensure that you have the correct spark plug for your vehicle.

Lead Erosion

Lead erosion is caused by lead compounds in the gasoline which react chemically with the material of the electrodes (nickel alloy) as high temperatures; crystal of nickel alloy fall off because of the lead compounds permeating and separating the grain boundary of the nickel alloy.

Typical lead erosion causes the surface of the ground electrode to become thinner, and the tip of the electrode looks as if it has been chipped.


Melting is caused by overheating. Mostly, the electrode surface is rather lustrous and uneven. The melting point of nickel alloy is 1,200~1,300°C (2,200~2,400°F).

Identifying Signs of Spark Plug Damage

#1. Look for light grey or tan metal on a good spark plug.

A spark plug that is not damaged will appear light grey or possibly tan. There should be no buildup on the sparking surface of the plug. If you purchased new spark plugs, their colors should be fairly similar.

  • A good spark plug will not need to be replaced.
  • If you have been having a misfire in the cylinder with a good spark plug, there may be an issue with the plug wire. Try replacing it to see if that solves the problem.

#2. Identify signs of carbon fouling.

Identify signs of carbon fouling. Carbon fouling often occurs when your vehicle’s engine is running too rich. Running too rich means there is too much fuel in the air/fuel mixture when it is ignited by the spark plug. Look for black soot on the sparking surface of the plug.

  • A black, dry soot may develop on the plug when the engine is running too rich.
  • You may be able to clean the soot off of the plug and avoid having to replace it. Wipe it down with a rag or spray it with Brake Cleaner to clean it.
  • You will need to bring your vehicle to a mechanic to diagnose why it is running rich.

#3. Check for wetness on an oil-fouled plug.

Check for wetness on an oil fouled plug. If there is oil leaking in your engine down past worn piston rings or through the valves on your cylinder head, the spark plug may be oil-fouled. Look for wet motor oil on the plug itself.

  • Oil fouled plugs will be wet, whereas carbon fouled plugs will be dry.
  • You will need to address the oil leak to prevent fouling more spark plugs.
  • An oil leak into the engine can cause significant issues. Bring your vehicle to a mechanic for diagnosis and repair.

#4. Look for blisters or burning.

Look for blisters or burning. If you have been having overheating issues with your engine, you may burn out a spark plug. Look for blistering on the insulator tip of the spark plug or signs of heat damage such as melted plastic or burned metal.

  • A heat damaged spark plug will have to be replaced.
  • Overheating issues may be caused by worn out or insufficient levels of coolant.
  • Add coolant if your vehicle is low, otherwise you may need to drain and flush your radiator.

#5. Check for signs of severe wear.

Check for signs of severe wear. A severely worn spark plug is simply a plug that has been in use for too long. Older vehicles that have never had their spark plugs replaced may experience spark plug failure without fouling them in the ways listed above.

Severe wear can cause a spark plug to come apart as you remove it. You may also see signs of eroded connections or cracking plastic. Severely worn plugs must be replaced.