How to Tell If a Spark Plug Is Bad – Symptoms and Testing Guide

A car’s engine is a complex system that relies on various components to function properly. One of these essential components is the spark plug – a small but mighty device responsible for igniting the fuel-air mixture in the combustion chamber. Over time, however, spark plugs can wear down and become less effective, leading to a variety of performance issues. Knowing how to tell if a spark plug is bad can save you time, money, and frustration in the long run. In this post, we’ll explore the signs of a failing spark plug, how to test it, and when to replace it to keep your car running smoothly.


The spark plug is a small but crucial component of your car’s ignition system. It plays a vital role in igniting the fuel mixture in the combustion chamber of your engine, and a bad spark plug can lead to a host of problems for your vehicle.

When a spark plug starts to go bad, it can cause issues with the performance of your car. You may notice rough idling or a decrease in fuel economy. And if left unchecked, a faulty spark plug can even cause your engine to misfire or fail to start altogether.

But how do you know if your spark plug is bad? In this article, we’ll explore the signs of a bad spark plug and offer tips on testing and replacing them. Whether you’re a seasoned mechanic or a novice car owner, understanding the basics of spark plug maintenance is essential for keeping your car running smoothly. So buckle up and get ready to learn all about spark plugs!

What Is a Spark Plug?

Parts of a Spark Plug

The spark plug may appear to be a small and straightforward component, but it has several parts that work together to generate the spark needed for combustion in your car’s engine. Understanding the different parts of a spark plug can help you diagnose issues and make more informed decisions regarding maintenance and replacement.

The three main parts of a spark plug are the center electrode, insulator, and metal shell. Let’s take a closer look at each one:

  • Center Electrode: This is the part of the spark plug that extends into the combustion chamber. It is responsible for conducting electricity from the ignition system to the ground electrode, which generates the spark. The size and shape of the center electrode can affect the performance of the spark plug, as well as the overall efficiency of the engine.

  • Insulator: The insulator is the ceramic piece that surrounds the center electrode and separates it from the metal shell. Its job is to prevent electrical current from flowing through the metal body of the spark plug, which could cause a short circuit. The insulator must also be able to withstand high temperatures and pressure without cracking or breaking.

  • Metal Shell: The metal shell, also known as the housing or ground electrode, provides a path for the electrical current to flow back to the ignition system. It is connected to the engine block, which is grounded, creating a complete circuit. The metal shell can be made of various materials, such as copper, nickel, or platinum, depending on the application.

In addition to these main parts, some spark plugs also include additional features, such as a resistor or multiple ground electrodes. These features can help to reduce electromagnetic interference and improve performance in certain applications.

Overall, understanding the parts of a spark plug can help you identify issues and select the right replacement for your car’s engine. By choosing the right size and type of spark plug, you can ensure efficient combustion, better fuel economy, and improved overall performance.

Signs of a Bad Spark Plug

Rough Idling

Rough Idling

One of the most common signs of a bad spark plug is rough idling. This occurs when your engine seems to vibrate excessively or even stall while it’s at idle. Idling is when your engine is running but your vehicle is not in motion, such as when you’re sitting at a stoplight or waiting in traffic.

Rough idling can be caused by a variety of issues, such as a dirty air filter, clogged fuel injectors, or a malfunctioning spark plug. In the case of a bad spark plug, the uneven combustion in your engine can cause vibrations that you’ll feel throughout your car.

If you experience rough idling, it’s important to have your vehicle inspected by a qualified mechanic. Not only can rough idling be annoying and uncomfortable, but it can also lead to more serious problems if left unchecked.

Replacing your spark plugs can often fix the issue of rough idling. However, if the problem persists after replacing your spark plugs, it could be a sign of a deeper underlying issue with your engine.

In summary, rough idling is a symptom of a bad spark plug and should be addressed promptly to prevent further problems. If you notice vibrations or stalling while idling, have your vehicle inspected by a professional to determine the root cause of the issue.

Poor Fuel Economy

Poor Fuel Economy

One of the most common signs of a bad spark plug is poor fuel economy. When your car’s spark plugs aren’t functioning properly, it can cause your engine to work harder than it needs to in order to keep running. This increased effort means that your car uses more gas than it should, resulting in a decrease in fuel efficiency and gas mileage.

Fuel efficiency refers to how much gas your car uses to travel a certain distance. The higher the fuel efficiency, the farther you can go on less gas. Gas mileage, on the other hand, is the number of miles your car can travel on one gallon of gas. Both are important measures of your car’s performance and can be impacted by a variety of factors, including spark plugs.

When a spark plug isn’t firing correctly, it can cause incomplete combustion in the engine. This means that some of the fuel doesn’t get burned, and instead is wasted. This wasted fuel results in decreased fuel efficiency and gas mileage. In addition, a malfunctioning spark plug can also cause the engine to run hotter, which creates additional strain on the entire system and can further reduce fuel efficiency.

To improve your car’s fuel economy and gas mileage, it’s important to regularly maintain and replace your spark plugs. By doing so, you’ll ensure that your engine is running efficiently and that all of the fuel is being used properly. Additionally, you can take steps like reducing your speed, removing excess weight from your vehicle, and avoiding rapid acceleration or hard braking to help improve your car’s gas mileage.

In conclusion, if you notice that your car’s fuel efficiency or gas mileage has decreased, it may be a sign of a bad spark plug. Regularly maintaining and replacing your spark plugs can help keep your engine running smoothly and improve your car’s overall performance.

Difficulty Starting the Engine

Difficulty Starting the Engine

Are you having trouble starting your car? One of the most common causes of difficulty starting an engine is a faulty spark plug. When a spark plug is worn out or damaged, it can prevent the engine from starting properly.

One sign of a bad spark plug is difficulty in cranking the engine. The engine may turn over slowly or not at all. This is because the spark plug is not providing the necessary spark to ignite the fuel in the combustion chamber. Without a spark, the fuel cannot ignite and start the engine.

Another cause of difficulty starting the engine is a problem with the ignition system. The ignition system is responsible for creating the spark that ignites the fuel. If there is a problem with the ignition system, such as a faulty ignition coil or distributor, the spark may not be strong enough to start the engine.

To diagnose the problem, it’s important to check the spark plugs and ignition system. You can test the spark plugs with a multimeter to see if they are producing the correct amount of voltage. If the spark plugs are fine, then the problem may be with the ignition system.

If you’re not comfortable testing the spark plugs and ignition system yourself, it’s best to take your car to a mechanic. They can perform a diagnostic test to determine the root cause of the problem.

In summary, difficulty in starting the engine is often caused by a faulty spark plug or ignition system. By understanding the symptoms and causes of this problem, you can take steps to diagnose and fix the issue. Don’t ignore difficulty starting your engine, as it can lead to more serious problems down the road.


Misfiring is a common issue that can occur with spark plugs, and it can manifest in several ways. One of the most noticeable symptoms of misfiring is when the engine begins to jerk or stumble while driving. This can be an unnerving experience for drivers, as it can feel like the car is about to stall or shut down completely.

Another symptom of misfiring is skipping, which occurs when the engine fails to fire on all cylinders. This can result in a loss of power and a rough ride, especially when accelerating or climbing hills. In extreme cases, skipping can cause the engine to stall out completely, leaving the driver stranded on the side of the road.

Misfires can be caused by a number of factors, including worn-out spark plugs, faulty ignition coils, or clogged fuel injectors. It’s important to address the underlying cause of the misfire in order to prevent further damage to the engine.

If you suspect that your car is experiencing misfires, it’s important to have it inspected by a mechanic as soon as possible. Ignoring the problem can lead to more serious issues down the line, such as damage to the catalytic converter or other components of the exhaust system.

In some cases, misfires can be resolved by simply replacing the spark plugs. However, if the issue persists, it may be necessary to replace other components of the ignition system, such as the ignition coils or control module.

Overall, misfiring is a serious issue that should not be ignored. If you’re experiencing any of the symptoms of misfiring, it’s important to address them promptly to avoid further damage to your engine and ensure your safety on the road.

How to Test a Spark Plug

Testing a spark plug is an important step in diagnosing any issues with your engine’s ignition system. Whether you’re experiencing poor performance or just performing regular maintenance, testing your spark plugs can help you identify potential problems before they become major headaches.

To test a spark plug, you’ll need either an ohm meter or a multimeter. An ohm meter measures the resistance of an electrical circuit, while a multimeter can measure voltage, current, and resistance.

Here’s how to use an ohm meter to test a spark plug:

  1. Remove the spark plug from the engine.
  2. Set your ohm meter to the lowest setting.
  3. Attach one lead to the metal shell of the spark plug and the other lead to the center electrode.
  4. The ohm meter should read between 0.4 and 2 ohms. If it reads infinity or a very high number, the spark plug is bad and needs to be replaced.

Here’s how to use a multimeter to test a spark plug:

  1. Set your multimeter to the lowest voltage setting.
  2. Attach one lead to the metal shell of the spark plug and the other lead to the center electrode.
  3. Have someone crank the engine while you watch the multimeter reading.
  4. The voltage should be between 5,000 and 20,000 volts. If it’s lower than 5,000 volts, the spark plug is bad and needs to be replaced.

It’s important to note that testing your spark plugs should be done on a regular basis as part of your engine’s maintenance routine. By doing so, you can ensure that your engine is running smoothly and avoid more costly repairs down the line.

When to Replace Your Spark Plugs

Manufacturer’s Recommendations

Manufacturer’s Recommendations

Following the manufacturer’s recommendations is an essential part of maintaining your vehicle’s health. The owner’s manual provides detailed information about your car’s specifications, including the recommended maintenance schedule. Typically, manufacturers suggest replacing spark plugs every 30,000 to 100,000 miles, depending on the type of spark plug and the make and model of your vehicle.

It is important to note that some manufacturers may recommend different replacement schedules for specific driving conditions. If you frequently drive in stop-and-go traffic or under extreme weather conditions, your spark plugs may need to be replaced more often than suggested in the standard maintenance schedule.

In addition to replacement schedules, the owner’s manual may provide additional recommendations for keeping your spark plugs in good condition. For example, it may advise against using aftermarket spark plugs or specify a particular type of spark plug that is compatible with your vehicle.

By following the manufacturer’s recommendations, you can ensure that your spark plugs are functioning correctly and avoid costly repairs down the road. Be sure to refer to your owner’s manual regularly and adhere to the recommended maintenance schedule to maintain your vehicle’s peak performance.

Visual Inspection

Visual Inspection

A visual inspection is one of the easiest ways to determine whether your spark plugs need replacing. All you have to do is remove each plug and examine it closely for wear or damage. Here are some things to look for:

  • Wear: Over time, the electrode on a spark plug can wear down. This can cause misfires and other performance issues. Look at the tip of the plug to see if it’s worn down. If it is, it’s time to replace the plug.

  • Buildup: If there’s too much buildup on the plug, it can cause problems with ignition. Look for any deposits or carbon buildup on the plug. If it looks like there’s too much buildup, it’s time to clean or replace the plug.

  • Cracks: Any cracks in the insulator or metal shell of the spark plug can lead to misfires or other problems. Look for any signs of cracking or damage on the plug. If you see any, replace the plug immediately.

  • Oil Deposits: Oil can get into the combustion chamber through a variety of sources, such as worn valve guides or piston rings. If you notice oil deposits on the spark plug, it could be a sign of bigger issues with your engine. Address those problems before replacing the plug.

It’s important to note that different types of engines and driving conditions may cause spark plugs to wear differently. Always consult your owner’s manual for manufacturer recommendations on when to replace your spark plugs.

By performing regular visual inspections of your spark plugs, you can catch any problems early and avoid more serious issues down the road. Don’t underestimate the power of a simple inspection – it could save you time and money in the long run.

Performance Issues

Performance Issues

When it comes to spark plug problems, performance issues are a common symptom that drivers may experience. The performance of your vehicle can be impacted in various ways when the spark plugs are not functioning properly.

One of the most noticeable symptoms of a bad spark plug is reduced engine power. This is because the spark plug is responsible for igniting the fuel in the combustion chamber, and if it is not firing correctly, the engine will not be able to produce as much power as it should. This can result in sluggish acceleration and poor overall performance.

Another performance issue that can occur with bad spark plugs is misfiring. Misfiring happens when the spark plug fails to ignite the fuel in the combustion chamber correctly. This can cause the engine to run unevenly, resulting in jerking, skipping, or even stalling. If left unchecked, misfiring can lead to more severe engine damage over time.

In addition to reduced power and misfiring, other performance issues that can indicate a bad spark plug include rough idling and decreased fuel efficiency. A spark plug that is not firing correctly can cause the engine to vibrate excessively at idle, leading to an uncomfortable driving experience. Decreased fuel efficiency can also occur when the spark plugs are not working correctly, as the engine may need to work harder to compensate for the lack of proper ignition.

Overall, performance issues can be a clear indication that something is wrong with the spark plugs. If you notice any of these symptoms, it is essential to have your vehicle checked by a professional mechanic to determine the root cause of the problem. Replacing the spark plugs may be necessary to restore your car’s performance and ensure that it is running smoothly.


After reading this guide, you should now have a better understanding of how to tell if a spark plug is bad. As we’ve discussed, a bad spark plug can cause a variety of issues with your car’s ignition and overall performance. It’s important to keep an eye out for warning signs such as rough idling, poor fuel economy, difficulty starting the engine, and misfiring.

Regular maintenance, including testing and replacing your spark plugs as needed, is key to keeping your car running smoothly. Following the manufacturer’s recommendations and visually inspecting your spark plugs can help you identify when it’s time for replacement. And if you experience any performance issues with your car, it’s always best to get it checked out by a professional mechanic.

Remember, neglecting your spark plugs can lead to more serious problems down the line and can even damage other parts of your car’s engine. So don’t ignore the signs of a bad spark plug – take action and keep your car in top condition.
After reading this article, it is clear that a bad spark plug can cause various problems for your car’s engine performance. The symptoms of a failing spark plug can range from rough idling to difficulty starting the engine, and misfiring. Regular maintenance and replacement of spark plugs are essential to ensure your car runs smoothly and efficiently. Testing the spark plugs and replacing them at the recommended intervals or when you notice any warning signs can help prevent costly repairs in the future. In summary, paying attention to your car’s ignition system, including the spark plugs, is crucial for keeping your vehicle running smoothly and avoiding potential breakdowns on the road.

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