What Do Spark Plugs Do – Everything You Need To Know!

What Do Spark Plugs Do – Everything You Need To Know!

Have you ever asked yourself what do spark plugs do? Well, if that is the case, then you are at the right place. We will learn everything about the plugs and learn what they do in an internal combustion engine.

Knowing how things work is not an essential thing to do, but it is quite useful. Why I’m saying this? Well, I’m saying this because when you know how some components work, you know how to diagnose them properly.

For example, let’s take the spark plugs. Whenever you are dealing with bad plugs they start to develop symptoms that will be present. And if you know how the spark plug work, you will be able to tell what is the problem and be able to uncover it relatively quickly. And by this were mean replacing the plugs of course. These are one of the components that you cannot refurbish. But more on that later on in the article.

First, we are going to learn what are these plugs and what do spark plugs do. Then we will cover the basic diagram of a spark plug and the symptoms of bad spark plugs. After that, we will learn how you can read spark plugs and uncover this problem in your car. So, if you want to learn more, follow us till the end because there will be a lot of useful information that we are going to share.

What Are Spark Plugs?

Now before we dive into more complex topics like learning what do spark plugs do and the symptoms of bad plugs. Let’s first focus more on the basics and learn what are these spark plugs in general. Why they are so special and widely used today? Let’s elaborate.

Spark plugs are one of the components that are used on cars since the early days of automobiles. These components haven’t changed a lot since their inception. As would some people call it, they were perfect from the beginning.

Jokes aside, they still got upgraded and refined throughout the years of use. Engineers have created ways to make them last quite more than they previously did and also make them cheaper in the process.

The early types of plugs were made out of copper. And copper was not one of the best metals to make a spark plug. These plugs didn’t last for a long time and owners had a lot of trouble later on.

So, something new had to be invented and this is how came the more modern platinum and iridium plugs. Especially the iridium plugs are one of the plugs that last for a really long time. You can get more insights into the differences in our guide on whether are all spark plugs the same.

But what makes a spark plug last for a very long time is the actual engine. So, how come? Well, the engine design is really important when it comes to the life of the spark plug. If your engine burns oil, burns coolant, runs too rich or too lean, and constantly overheats, you will have a ton of problems with spark plugs.

Modern engine designs rarely do any of this stuff. Thus, making spark plugs last for a really good time. Now let’s learn what do spark plugs do.

What Do Spark Plugs Do

So, we covered what are spark plugs and the different types of spark plugs that exist out there. Namely, there were copper, iridium, and platinum plugs. But what do spark plugs do?

Well, the answer to that question is quite simple. They produce a spark and ignite the fuel inside of the cylinder. Then this ignition creates the explosions that move the pistons up and down in the block.

Without a spark, the engine will not be able to work at all because the fuel will not get ignited properly. Fuel could also be ignited by compression.

And here you need to learn an important thing. Gasoline is always ignited by a spark. Diesel fuel is ignited by compression. Which in other words translates that only gas-powered vehicles have spark plugs.

There is a single spark plug for each of the cylinders. For example, if you have an inline-4 engine you have 4 plugs. If you have a V8 engine, you have 8 plugs.

But, there could also be engines that run twin sparks. These engines use two spark plugs per cylinder. Two plugs are also used on some vehicles.

For example, a 5.7 V8 HEMI is running two plugs per cylinder. These engines are rare but you need to be aware of them that they do exist.

What is good is that you get more power since you get better ignition. But if one or more of them fail, you will have a ton of bad spark plug symptoms that we are going to cover later on. Overall, that is all you need to know when it comes to what do spark plugs do. Now let’s see from what parts are these spark plugs made.

Parts Of A Spark Plug

Now as we learned what do spark plugs do, let’s now focus more on the internals of the spark plug and understand how spark plugs work in general.

Spark plugs are also known as electrodes. Their job is to transfer high current. So, their structure is built in a specific way to make this process possible.

At the top there is the terminal, the terminal is the component to which you connect the spark plug from the top.

What Do Spark Plugs Do

On the outer layer of the spark plug, there is a ceramic insulator and inside of the insulator, there is a resistor, suppressor (or spring in other words), and the electrode in the center.

There is also another outer layer which is the shell, treads, and the ground electrode. The ground electrode is essential because you need a ground in order for the spark plug to work and produce a spark.

Overall, this is the basic diagram or in simple words, the parts of a spark plug. There are many of them. But you shouldn’t worry much about them except for the electrodes. The electrodes have to be in good condition if you want your spark plugs to perform as they should.

But more on this, we are going to cover later in the article, where we will learn how you can read spark plugs. Now let’s cover more on the benefits of replacing the plugs.

Benefits Of Changing Spark Plugs

Now as we cleared what do spark plugs do and what are the main components inside of the spark plug, let’s now focus more on the benefits of changing spark plugs. But before this, let’s learn more about how long should spark plugs last in the first place.

When it comes to spark plugs, it is worth noting that these components on modern engines last for a really good time.

Back in the day, they lasted about 20,000 or even less. Mainly because older engines performed quite poorly and were not that refined. Spark plugs got easily fouled and broke quite often.

Modern engines can easily push a set of spark plugs for 60,000 miles. And in some cases, they can even go for 100,000 miles with no problem. Especially high-quality plugs that are made out of iridium. But is there any benefit of changing spark plugs? Let’s elaborate.


When it comes to the pros of changing your spark plugs, we can say that there are a few of them. The first reason is that you will not suffer from the symptoms of bad plugs that we will discuss next.

And the second reason is that you will get a good performance out of your car. The fuel will burn a lot better with new plugs and the engine will start to feel more alive and will be more responsive. And this is how every engine should perform if you are asking me.

This is why even if the spark plugs are fairly good, you can just replace them if they are old or have a lot of miles on them. So, we covered what do spark plugs do and why you should replace them, now let’s move to other topics.

Symptoms Of Bad Spark Plugs

Now as we cleared what do spark plugs do, we think that now it is time to discuss more on the symptoms of bad spark plugs.

As you probably know, every component when it starts to fail it develops certain symptoms that are present. And so is the case with the spark plugs. They produce a number of these symptoms and we are going to discuss all of them in the next couple of chapters.

1. Check Engine Light

We covered what do spark plugs do, now let’s move on to the first symptom of bad spark plugs. And that symptom is the check engine light. So, why is this the case, why does this light illuminates when these components are broken?

Well, this light illuminates when the PCM/ECM detects misfires in a certain cylinder. So, if cylinder number 3 is misfiring, you will get a code, more precisely the P0303 code. Or possibly a different code that ranges from P0300 to P0308. Depending on which cylinder is affected or if it’s a random misfire.

What Do Spark Plugs Do

Having an OBD2 scanner will be one of the best investments in this situation. And you can easily diagnose these components like plugs or coils. Just swap them out with ones from a different cylinder that doesn’t misfire and you will be able to tell if the spark plug or coil is bad for the specific cylinder.

We learned what do spark plugs do and the first symptom, later on, we are going to cover the diagnosing process.

2. Engine Doesn’t Want To Start

Now as we cleared what do spark plugs do, we can move on to the next symptom, which is the situation when the engine cranks and doesn’t start or starts after heavy cranking.

This situation at first glance could indicate a battery-related issue or possibly an issue. But you shouldn’t jump to conclusions because the spark plugs can create a similar scenario.

When the plugs get old, they get out of shape. They produce a weaker spark or no spark at all. So, what happens is that the engine simply has trouble starting and running in general.

What Do Spark Plugs Do

These plugs are really essential components that make the engine work as it should. If they are not in good shape, the engine will not want to start and you will experience issues like this.

So, learn what spark plugs do, and learn the symptoms. If this is one of the symptoms that you are dealing with, replace the spark plugs and this will hopefully solve the problem. Now let’s move on to the next symptom.

3. Engine Misfires

Now after we learned what do spark plugs do, let’s cover the next symptom concerning the bad spark plugs. And this is the situation when you experience engine misfires.

Engine misfires are very common on engines that have fouled plugs. They are often even reported as trouble codes, something that we already explained. But what is a misfire?

Well, a misfire is a situation when the engine is not working as it should. More precisely, this is the situation when the explosions inside the cylinder are either premature or delayed. So, this situation creates a ticking noise that comes from the top of the engine.

Also, from the exhaust, you will notice pops and bangs. The gas flow will not be equal all the time. And when this happens to an engine, you definitely have an engine that is misfiring and you should learn what do plugs do and sort out this problem as soon as possible. But more on that later on, now let’s cover the next symptoms.

4. Rough Engine Idle

As we cleared what do spark plugs do, let’s now cover another symptom of bad spark plugs. And that is the situation when you have a rough engine idle.

The rough engine idle is basically a consequence of the engine misfires. When the engine is misfiring, it also runs roughly in most cases.

Engine misfires ignition components diagnose

So, if the engine is shaking and the needle of the tachometer fluctuates all the time, you are dealing with rough idle. The engine will dump more fuel to recompense for these misfires and will cause the engine to run rich and the situation will get even worse. This is why you should not ignore a situation like this.

You need to learn what do spark plugs do and then move on to diagnosing stuff. Later on, we are going to explain how this work is done, now let’s cover the last symptom in our list of symptoms.

5. Engine Low On Power

Since we learned what do spark plugs do, now it is time to finish our list of symptoms. And we will finish it with the most common one. And that is the situation when the engine is down on power.

Whenever you are having problems with the plugs, the engine will be really down on power. You might feel like your engine is running on 30hp less than it did before when the plugs were in good condition.

The acceleration will be quite poor and the engine will not want to pull. It will basically be comparable with a low compression engine and will want to stall a lot.

This is why when you notice how the engine is down on power, you need to learn what do plugs do and how to tackle this problem. And in the following chapter, we are going to learn how you can diagnose bad plugs.

Spark Plug Diagnosis

Now as we cleared what do spark plugs do and all the symptoms associated with this problem. We need that now it is time to learn how you can diagnose them. First, we will learn how you can do this professionally with an OBD2 scanner and a multimeter. Then we will learn how you can tell if a spark plug is bad based on the color chart.

So, whenever you notice the engine misfires and the engine is down on power, it will be really useful to have an OBD2 scanner. Even if you don’t get the check engine light, you can still check if you have codes.

If you notice codes that are associated with misfires, you know where to look. Just find the cylinder that is misfiring based on the code and then swap out the spark plug with one from a different cylinder. If the misfire moves to the next cylinder, then you have a bad spark plug.

Another way that you can try to diagnose plugs is to measure the resistance of the spark plug. And you can do this with a multimeter. This tool will allow you to measure the resistance on the central electrode. A good spark plug should produce between 4,000 and 8,000 ohms. More on how you can test your spark plugs, you can check in the video.

Spark Plug Color Chart

Now let’s learn how you can compare bad spark plugs vs good ones based on the spark plug color chart. This is one of the most important things you need to know besides knowing what do spark plugs do. So, how you can read plugs?

Well, it is fairly simple. Just remove the spark plug and inspect it closely. A good spark plug should have a somewhat clean electrode. Also, the electrode should not be damaged. If you have some kind of debris or the electrode is damaged and missing a piece your spark plug is fouled, unfortunately.

There are a few ways that a spark plug can foul. The first is by running rich or lean. If the car is running rich it will be black and smell like gasoline. Lean on the other hand will cause blisters on the electrodes.

Oil consumption will also foul the spark plugs and cause them to be all black and have a ton of deposits. For more in-depth information, you can check the video that we attached above.

Spark Plugs Cost

So, we learned what do spark plugs do, now let’s cover the cost. How expensive is a set of plugs? Well, this really depends on how many of them you need. A good quality spark plug can cost $10 or even less. So, it is up to you how much you want to spend. The most you can spend is $120 on good-quality plugs.


In this article, we have covered quite a lot when it comes to spark plugs. First, we learned what they are and what they do. Then we learned more about the diagram and what components they are made of.

After that we covered all of the symptoms associated with these components and how you will be able to diagnose a car with bad plugs and lastly, we learned how much money can you expect to pay for a new set.


Now let’s answer some frequently asked questions.

How Long Do Spark Plugs Last

On average, they last about 60,000. But this number you should take with a grain of salt. Namely because if you have a certain condition in the engine like oil burning, coolant leaking, running rich or lean. All these states can quickly destroy a good plug. This is why you need to address any issue before you replace them.

How Much Are Spark Plugs

They are fairly cheap, you can find them for $10 apiece or even cheaper for a quality plug. So, when it comes to replacing them, you should never go cheap and clean them since even if you clean them, they will highly unlikely be working as they should if not replaced.

How To Check Spark Plugs

You can only check them by removing them from the cylinder. Remove them and inspect them for damage. Check the electrode, and see what kind of deposits are on there. If the plug is looking good, measure the resistance with a multimeter. A good plug should make between 4,000 and 8,000 ohms.

What Do Bad Spark Plugs Sound Like

They don’t produce sounds. But they could cause a situation that is known as an engine misfire. And these misfires can sound like small ticks coming from the top of the engine right above the cylinder that is misfiring.

What Does Changing Spark Plugs Do

Changing them will rejuvenate your engine completely. Especially if we are talking about old plugs that are fouled by oil, gas, and whatnot. Replacing them will give your engine a new life and will produce more power.

Do Diesel Engines Have Spark Plugs

No, diesel engines do not run plugs. The ignition of diesel is not done with the help of a spark. But with the help of compression. The fuel is compressed so much that it eventually ignites and starts the engine.

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