How to Tell If a Spark Plug Is Bad?

Driving a four-wheeler can become an important part of your life. However, there are numerous different components of the machine you should inspect over time. And how to tell if a spark plug is bad should be on the list of questions you should know the answer to. After all, as a car owner, you should be able to know when your vehicle is not working properly and what could be the main reason for it.

Faulty spark plugs can cause many issues in the car’s engine, and if not fixed on time, you’ll have a lot more problems than just replacing the plug. And in order to prevent it, you should be able to recognize the signs, which are a rough idle, a check engine light, a misfiring engine, high fuel consumption, and a loud engine.

What Are Spark Plugs?

Spark plugs are a crucial part of having an engine run properly for most cars. They are a company part of the complete engine running process, and without them, a vehicle won’t start, or the engine could have some malfunctions. Of course, not recognizing or ignoring some of the signs could lead to more significant problems with your four-wheeler. 

So, if you neglect them, it can cause much more problems than you might think. After all, they are crucial maintenance items and play a big role in the performance of an engine, exhaust emission standards, and fuel mileage. Of course, it doesn’t mean your work is done with this. On the contrary, while you’re all into inspecting the car, replace U-joints, check all of the lights, and change a tail light if needed. 

Plugs are an essential part of the vehicle’s engine

There Are Diverse Types You Should Know About

Most people believe that every plug is the same. However, people who are a little more knowledgeable about the performance, efficiency, and ignition system are aware that not all plugs are created equal. In fact, there are several varieties of these gadgets, despite the claims of numerous auto parts stores to the contrary. For instance, copper spark plugs have the shortest lifespans, whereas spark plugs composed of more sophisticated and long-lasting materials, like platinum or iridium, can last up to four times as long as copper ones. 

You should select the products that your car’s maker suggests. But let’s examine the specifics of each option and help you determine their suitability for your car.  


The platinum plugs (I recommend the Autolite  brand) have a platinum disc in the centered electrode merged to its tip area. This way, a plug is more durable and will last 100,000 miles. Also, more heat will be generated and reduce debris buildup. However, these plugs aren’t the strongest you can find on the market.

Double Platinum

When a car has a distributor ignition system and waste spark system, then installing double platinum sparks is more than recommended, especially the Bosch Automotive ones. The reason is that this system will make spark plugs fire twice – in the compression and exhaust stroke cylinder.


This kind of plug is made of solid copper, and nickel alloy is the main material of the central electrode. And since nickel alloy isn’t very durable, it won’t last as longest as other plugs. That’s why using them on older vehicles with less electrical needs is best. In this case it’s best to use the Autolite brand.


The most durable spark plugs are made of iridium. I always use Autolite brand. Iridium is a metal that is more durable and powerful than platinum. Despite the higher cost of iridium spark plugs, you get what you pay for.

Due to the small size of the core electrode, these spark plugs generate electricity at lower voltages. Because of this, many automakers are started to recommend iridium ones for their vehicles. If you already have all of these installed and need new ones, do not switch to platinum or copper because doing so will lower your car’s performance.

How to Tell If a Spark Plug Is Bad?

Cleaned and not damaged engine spark plugs are the results of your vehicle’s well function. Also,  if you want the optimum performance of your car, these have to be in top-notch condition. Otherwise, when a plug is dirty or fouled over time because of overheating, combustion or contamination, these components could lose their most important function – sparking the air-fuel mixture and dispersing the heat.  

All this, as you can assume, could affect the engine efficiency. In fact, if just one plug isn’t healthy, your four-wheeler won’t accomplish its potential and could show many other malfunctions. And nobody wants that to happen, right? 

As every component in your ride can become less effective due to wear, time, and many other ignition/electrical issues, the spark plugs are not different. However, some components will tell you directly they aren’t working properly, like an alarm, and you can proceed to disconnect it. Others will show you more indefinitely. So, check what symptoms may show you the spark plugs are faulty and need replacement:

  • A rough idle,
  • A check engine light,
  • Misfiring engine,
  • High fuel consumption,
  • Loud engine.

A Rough Idle 

The idle of a car is when the engine has started, but the vehicle is not moving, the engine is idling, and your foot is not on the gas. Now, if your idle is jumping up and down a little bit, that surely is a sign that new spark plugs are needed (or at least a new spark plug). In this case, your car is misfiring, or at least it’s not starting properly. 

This is the best test you can do to see whether your four-wheeler needs a new spark plug – start your engine, let it warm up, and then just watch the tachometer that will show you the RPMs (revolutions per minute) and see if it’s pretty stable or it’s jumping up and down. A tiny movement is perfectly normal, but you don’t want to see it moving significantly. 

man in a car at night
Start your ride and let it warm up before going on a trip

A Check Engine Light

When a plug is starting to fail or has failed, one of the most common things you’ll notice is a check engine light. Upon further diagnostics of that, it will show some more codes called ”p0300” all the way up to maybe ”p0306” depending if an engine is a six-cylinder or all the way up to ”p0308” if it’s an eight-cylinder. Those are also indications of a misfire which is caused by a bad plug.

If the check engine light is on, know the plugs might need to change
If the check engine light is on, know the plugs might need to change

The Engine Is Misfiring

The most common reason why an engine misfire is at least one bad plug. Of course, there are many other reasons that could lead to this problem, like inferior quality fuel or a bad ignition coil plug cap. If that’s the case, a driver should notice the pace falters of the engine. This practically means the fuel is not being processed, and the raw material is being sent to exhaust, which damages the catalytic converter.

You'll have to replace spark plugs when the engine is misfiring
You’ll have to replace spark plugs when the engine is misfiring

High Fuel Consumption 

If your car is consuming a lot more fuel than usual and if you’re getting low gas mileage (significantly lower than normal for your vehicle), it’s a good sign you should replace one or more spark plugs. And no matter whether they are dirty or malfunctioning 100% properly, it can cause your four-wheeler to use a lot more gas than usual. 

Seeing numbers as high as 30% higher than average fuel consumption is not abnormal or not unusual for needing new spark plugs. But, if you notice a huge amount of fuel consumption and you notice something is different, it’s a sign you might need a new spark plug. 

Needing more stops to a gas station than usual is a sign for replacing a plug
Needing more stops to a gas station than usual is a sign for replacing a plug

Your Vehicle Has Trouble Accelerate

Another sign is lower than normal acceleration. If you’re driving a car that has 120 horsepower and you feel like it is lacking in acceleration, it might be because of the horsepower number. If you’re used to driving the 200-horsepower one, and then you drive the 120-horsepower one, normally, there will be a decrease in acceleration. What you’re looking for is a decrease in acceleration from the same car when it was brand new. 

If you drive the same vehicle for five years, and you feel like it’s not accelerating nearly as well as it did before, that is a telltale sign you’ll need to replace spark plugs. However, keep in mind it’s completely normal for a vehicle to lose acceleration as time goes on slightly. But if significant changes are going on, you should look into getting a new spark plug.

 If a ride can't accelerate as before, it's a red flag
 If a ride can’t accelerate as before, it’s a red flag

Engine Is Noisier Than Before

Noise can be a great indicator that something is wrong with the spark plugs, no matter if you’re driving a car with a four, six, or eight-cylinder engine. In fact, if you notice some strange noise while driving, such as rattling, the first thing to look for is a plug. 

When the engine changes its sound, check each spark plug
When the engine changes its sound, check each spark plug

Should You Replace All Spark Plugs and How Often?

Many sources suggest replacing the entire plug set instead of only the bad one, or however you need. For example, once you have one plug replaced, the remaining will still be old ones and, more likely, will go bad soon. Another reason surely is the process – you won’t spend much more money on replacing the whole set, so why not do it then? 

Another thing to have in mind when you figure out the plug is bad is to get it replaced as soon as possible. They are an essential part of the engine, and the cost of replacing them is rather low. So, that being said, it won’t completely cost you a fortune, and it won’t get you in debt, compared to replacing a transmission or a head gasket blowing. 

Can You Fix It on Your Own and What Tools Will Be Needed?

This is the perfect job for people who have just started fixing cars on their own. With the right tools, it can be quite an easy process to do at home, save money and get your vehicle running in tip-top shape. Of course, it won’t be as easy as changing a flat tire or, on the other hand, maintaining your vehicle clean inside with clean seat belts or maybe a steering wheel, but once you know all the hacks, you’ll do it effortlessly. All you have to do is have the right tools:

Responsible Driver Is the Best Driver

Noticing early warning signs is surely a crucial part of being a responsible driver. So, pay attention to your four-wheeler’s behavior, and as soon as you notice something is different, have it checked. It’s very easy to overlook some little changes, and it’s completely normal. That’s why you must get your car checked and have a routine maintenance service. Otherwise, you’ll be dealing with much bigger problems than just replacing a faulty spark plug.