Good Spark Plug vs Bad Spark Plug—How to identify Them

Spark plugs are an important component of your ignition system that helps start your vehicle. They do this by igniting the fuel-air mixture in the combustion chamber.

However, to function effectively, they must be in a certain condition. You can ascertain this by looking at them. You only need to pull them out hold, and inspect them to check if it is okay or has gone foul. Good spark plug vs bad, how do you identify them? Read on to learn how to single out good spark plugs from the bad ones easily.

Good spark plugs vs bad spark plugs comparison table.

Good spark plugs Bad spark plugs
The plugs look fresh The plugs are carbon-fouled.
There is no oil Presence The plugs are oil-fouled
Leads are in good condition The plug leads are fouled
No burning signs The plugs look burned
Electrodes are ok Electrodes are worn
There is no broken sign Electrodes are broken
The spark plugs are gapped correctly There is an uneven spark plug gap
Engine runs smoothly Engine misfires or has starting problems
Better fuel economy Higher fuel consumption

What does a new spark plug look like?

new spark plug

A new spark plug wouldn’t have any sediment in its body, electrode, or insulator. However, if you have used the plugs for a while, here is how to know if they are still in good condition. A good spark plug should have a brownish or light deposit at the electrode side, i.e., the plug’s firing end.

Sometimes, you may also notice that your spark plugs are wet. Plugs can get wet due to engine flooding. While this doesn’t mean they are bad, using it this way may impact engine performance. Allow them to dry out before reinstalling; otherwise, clean them.

For people wanting to know how should bad spark plugs look? Here you go.

What does a bad spark plug look like?

Bad spark plug

The electrode or insulator tip of a bad spark plug will look burnt, fouled, worn, broken, or covered with oil deposits. This cuts across other automobiles. So, if you’re wondering how to tell if the spark plug is bad in a snowblower or other vehicle, you may find this helpful.

Carbon-fouled plugs: Carbon-fouled spark plugs are plugs whose electrodes and insulator tips are covered in dry or black soot. A plug can get fouled due to dirty air filters, using rich or lean fuel, idling too long, or driving at a relatively low speed. Some symptoms of black spark plugs may include higher fuel consumption and sluggish acceleration, among other things.

Black or carbon-fouled plugs don’t burn fuel properly, resulting in longer acceleration. The inability to properly burn fuel also leads to insufficient engine power. The engine, however, compensates for lack of power by pushing more fuel into the combustion chamber.

Oil-fouled plugs: Oil-fouled plugs are plugs whose electrodes or insulator tips are covered with black oily deposits. This can happen due to oil leaking into the cylinders and getting past damaged pistons or valve guides.

Fouled plugs failing to ignite the fuel properly often cause an unusual vibration when starting the engine. While replacing spark plugs should handle the situation, find the source of the issue, and tackle it.

Lead-fouled plugs: In lead-fouled plugs, the nose of the insulator is covered in yellow deposits. This would also need replacement.

Burned plugs: Burned spark plugs are plugs with melted electrodes or blisters or white deposits on the electrodes due to running extremely hot. Spark plugs can get burned for several reasons, including incorrect heat range, engine overheating, and wrong ignition timing. Others include lean or rich fuel and loose plugs. Those asking what do bad spark plugs look like? Here is one.

Worn electrodes: If the firing end of the spark plugs looks worn, it’s a telltale sign your plugs have lived past their life span and are due for a replacement. Old spark plugs may not be able to provide sufficient spark needed to start your car.

Aside from looking, another way to identify old or bad spark plugs is to look out for noise. What does a bad spark plug sound like? A bad plug produces a sputtering noise when starting your engine. You may also notice that your engine takes longer to start.

Broken electrodes: Electrode breakage can occur due to thermal expansion or heat caused by an abrupt cooling or heating. Plugs with broken electrodes are also an indication that you installed the wrong spark plugs.

While shorter plugs cause poor fuel economy and plug fouling, longer ones can damage the engine. Remove and install the correct spark plug for your specific vehicle.

Wrong spark plug gap: While this isn’t necessarily classified as a bad spark plug, it can hamper your engine’s performance. The spark plug gap is the distance between the spark plug tip and the central electrodes. Spark plug gap in cars is usually between 0.6-1.8mm, depending on your vehicle.

If your plug is too big or small, there will be inconsistency in the spark moving between the ground and the central electrode. This often results in poor fuel economy and rough idling and sometimes causes the engine to fail.

To get the best spark plug gap for your specific vehicle, refer to your owner’s manual. While you may know how to check spark plug gap, adjusting is another thing. If you don’t know how to adjust plug gaps, reach out to your mechanic.

Learn More: How To Read Spark Plugs?

Good spark plug vs bad—what are their differences?

good spark plug vs bad spark plug

The major differences between good and bad plugs are their look and how they perform.


A good and functioning plug will have only brown or light grayish deposits on the plug electrode. As for a bad plug, it tends to have wear or dark deposits on the electrodes or insulators.

Still wondering, what do bad spark plugs look like? Pull out the plugs, hold them with your hand, and inspect them. If the electrodes appear broken, torn, or have black residues, then the plugs are bad. Your mechanic should tell you what action to take —cleaning or replacement.


A good plug provides good starting power and optimal engine performance. With a bad plug, you will experience decreased mileage and engine misfires. You may also notice a sputtering or rattling noise or stalling when the spark plug makes the engine misfire.

Engine misfires can also result from bad oxygen sensors or a clogged fuel filter. To be sure, misfires stem from a bad plug. So, conduct a ground or multimeter test to see if the engine is firing well. If you don’t know how to check if spark plugs are firing properly, reach out to your mechanic.

Though other things can lead to the illumination of the check engine light, engine misfire is also a culprit. To be certain, diagnose with an OBD II scanner. Engine misfires or bad spark plugs should show codes ranging from P0-300 to P0-308.  In a worst-case scenario, you will have problems starting your car.

New SP vs. Old SP: Mechanic Discussion

I understand the need to have a discussion with different mechanics and car owners where everyone shares their ideas and ordeals about a specific topic. I might not cover everything you need to know. But trust me, this article is comprehensive enough to let you know the differences between good and bad spark plugs.

But if you are wondering about the differences between good and bad plug performance on a specific car, read this Reddit discussion. The discussion started four years ago with 110 comments about good and bad plugs on different cars.

Final Words

By now, you should be able to differentiate between a good spark plug and a bad one. I have explained how a good spark plug should look—including how to tell if spark plugs are bad by looking at them. You can easily tell the condition of your plugs by removing and examining each of the plugs.

Aside from inspection, watch out for bad spark plug symptoms like engine misfires, delayed acceleration, poor fuel economy, etc. If you notice any of these signs, replacing bad plugs should get your engine working well again. If you are unsure what spark plugs suit your engine, contact your mechanic or check your owner’s manual.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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