How To Check If a Spark Plug is Bad: 4 Steps Process

A bad spark plug on hand

Spark plugs are an essential part of your car. They ignite the fuel in the engine, and if they are bad, it can cause all sorts of problems. In this post, I will give you a step-by-step guide on bad spark plug symptoms. If you follow these instructions, you can determine whether or not a spark plug needs to be replaced!

What Are the Consequences of a Bad Spark Plug

a Bad-Spark-Plug
Image: YouTube / Scotty’s Neverending Odyssey

If your spark plug is bad, it can cause a few different consequences. One consequence is that your car will have a hard time starting. This is because the spark plug is responsible for igniting the air/fuel mixture in the engine, and if it’s not working properly, the engine won’t start.

Another consequence of a bad spark plug is reduced fuel economy. This happens because the spark plug helps to ensure that the air/fuel mixture in the engine is burning completely, and if it’s not burning completely, you’re wasting gas.

Finally, a bad spark plug can also lead to increased emissions from your car. So not only will your vehicle be less efficient, but it will also be polluting more. So, if you think you have a bad spark plug, it’s best to take your car to a mechanic and check it out. Don’t wait until it’s too late and you’re stuck on the side of the road with a car that won’t start.

4 Steps of Checking Spark Plug

Image Credit: Engineering Explained

Bad spark plugs can cause engine problems, from misfires to hard starts. Checking the spark plugs is a good place to start if you’re having trouble with your car’s ignition system. Now, I will show you how to check a bad spark plug.

Step 1: Removing the Spark Plug

First, remove the spark plug wire from the spark plug. You can do this by pulling on the boot (the rubber part that covers the metal connector) or using a pair of pliers. Be careful not to damage the boot as you pull it off – if it’s torn or damaged, it will need to be replaced.

Step 2: Cleaning Spark Plug

Once the boot is off, look at the end of the spark plug. If it’s covered in oil or debris, that’s a sign that the spark plug is bad. Then, clean the spark plug with a wire brush and re-attach the boot.

Step 3: Initial Test

If the spark plug looks clean, put it back in the engine and start the car. If the engine runs smoothly, the spark plug was probably just dirty and didn’t need to be replaced. However, if the engine still misfires or has trouble starting, you need to test the spark plug for its resistance. 

Step 4: Resistance Check

To check the resistance of your spark plug, you need to use a multimeter to test the resistance of the spark plug. For example, you can get an idea of the resistance of your spark plug from the spark plug resistance chart. The superior resistance should be within a specific range, depending on the spark plug type. The spark plug is likely bad if the resistance is outside this range, and then you might need to replace your sparkplug.

By following these steps, you can ensure that your spark plugs are in good condition and are firing correctly. This will help to keep your engine running smoothly.

The Benefits of Checking Spark Plug

It is essential to check your car’s spark plug regularly to ensure that it is operating efficiently. There are several benefits to doing this, including:

  1. Improved fuel economy – Checking and changing your spark plugs can improve your car’s fuel economy by up to 30%.
  2. Reduced emissions – A well-maintained spark plug will produce fewer emissions, helping you do your bit for the environment.
  3. Enhanced performance – Regularly checking and changing your spark plugs can enhance your car’s performance. You’ll notice a difference in how smoothly it runs and how much power it has.

So there, you have three good reasons to keep on top of your spark plug maintenance! First, do it yourself or take it to a professional – either way, it’s worth doing.


So, this was all about checking a bad spark plug. A bad spark plug can lead to several problems with your car, from decreased fuel efficiency to complete engine failure. Fortunately, checking if a spark plug is bad is a relatively straightforward process and one that you can do yourself without any special tools or expertise.

By following the simple steps in this guide, you can quickly determine whether or not your car’s spark plugs need to be replaced.

Thanks for reading!


How to tell if a spark plug is bad by looking at it?

If you look at your spark plug and see that it’s black, sooty, or oily, then you can know that your spark plug is bad.

How long do spark plugs last?

Spark plugs last between 80000 to 100000 miles; after that, you need to replace your spark plug.

Are spark plugs expensive?

The answer may depend on who you ask, but typically, they are not too pricey. A single spark plug can range in price from a few dollars to upwards of $10. However, remember that most cars require 4-6 spark plugs so the overall cost will be higher.

Additionally, some luxury vehicles may require special or premium spark plugs that can cost even more. Nevertheless, spark plugs are relatively affordable compared to other car parts and maintenance costs.

What is a spark plug resistance chart?

A spark plug resistance chart is a table that shows the correct resistance value for each type of spark plug. This information ensures that the correct spark plug is installed in the engine. Incorrectly installed spark plugs can cause engine damage or poor engine performance.

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