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Rough Idle & Misfires - Is It Your Spark Plug or Ignition Coil?

Technical  ·  September 15, 2021

Rough Idle & Misfires - Is It Your Spark Plug or Ignition Coil?

Cars have all sorts of problems, but what most people experience are rough idling or engine misfires. These problems can be traced to your vehicle's ignition system, but the root cause isn't always so clear. That's because the same issues can be caused by either spark plugs or ignition coil. 

So, how do you know which one is the culprit?

Spark plugs and the ignition coil are both parts of the ignition system. When they go bad, they can cause rough idling or engine misfires. Since they’re two separate components, you must diagnose them separately to determine which one is causing the problem. You can do that by using a spark tester, a multimeter, or by swapping parts.

In this article, we’ll explore how your spark plugs and ignition coil cause engine misfiring and rough idling and how you can figure out which part needs to be replaced.

Let’s get to it!

How to Tell Whether It’s Rough Idling or Engine Misfire 

As modern as cars are today, they still produce many different noises and vibrations as part of their regular operation. For many people, that can be unsettling. So, let’s start this guide by defining each of these symptoms.

What Does It Mean When a Car Is Rough Idling?

To idle means to have the engine running but the car is in place and in ‘Park’ or ‘Neutral’ gear. In more technical terms, the engine is running, but the transmission isn’t engaged, so you’re not going anywhere. The RPMs should be steady while your car stays in place, producing a gentle hum. 

However, if you’re idling and the RPMs start jumping up and down, and you feel shaking and random jerking, then that’s rough idling. These symptoms are not usual, and they’re a clear sign that there’s an issue requiring attention.

What Does It Mean When an Engine Misfires?

The entire combustion process happens inside your engine. Inside each cylinder, air and fuel compress and combust several times a second. It sounds chaotic, but everything happens like a well-choreographed dance.

Engine misfires occur when that ‘dance’ starts to become chaotic. Combustions don’t happen when they should, and fuel doesn’t get burned correctly. Worst, all moving parts are no longer synchronised.

From the driver's seat, this will feel very much like rough idling – the RPMs go up and down erratically. With the car running, driving becomes unstable.

How Faulty Spark Plugs and Ignition Coils Cause Misfires & Rough Idling

Spark plugs and ignition coils are two separate components, but both belong in your car’s ignition system, at least on petrol-powered vehicles. The challenge here is that both parts create similar symptoms when they fail.

Let’s take a closer look at each one.

What Is a Spark Plug & How Does It Go Bad?

A spark plug is a component that fits into your engine, and there are several of them depending on the number of cylinders in the engine. As the name suggests, a spark plug ignites or ‘sparks’ the air-and-fuel mixture in the cylinder to start combustion.

Each spark plug fires hundreds of times per minute while the engine is turned on, whether it's idling or running.

Spark plugs can go wrong in several ways, but the most likely reason is natural wear and tear. They can also suffer from overheating, carbon build-up around the electrode, and other faults.

When spark plugs go bad, you’ll notice signs like:

What Is an Ignition Coil & How It Goes Bad?

The ignition coil, on the other hand, is a type of electrical transformer. It’s responsible for converting the battery’s low voltage to the thousands of volts necessary for spark plugs to ignite the fuel-air mixture. 

So, as you can see, spark plugs and ignition coils work closely, though they’re two separate components.

failed ignition coil can cause a lot of problems, including:

At this point, it’s quite clear that finding out the culprit is going to be a challenge because, for the most part, the tell-tale signs are the same.

So, how do you or your mechanic figure out which one is to blame?

Rough Idle and Misfires - Is It Your Spark Plug or Ignition Coil?

How do you tell whether the spark plug or ignition coil is to blame for almost identical symptoms? Well, you’ll have to get your hands dirty on this one and inspect the parts up close. You can do so by using specialised tools like a spark tester or testing lights, a more common tool like a multimeter, or the old-fashioned way of switching out parts and seeing if they work.

Let’s take a closer look at each option.

Spark Tester or Test Lights

Spark testers are specialised tools that help you diagnose both your spark plugs and ignition coils simultaneously. These testers typically have test lights on them that will flash if your plugs are indeed generating sparks as they should.

This method is the most straightforward and time-saving option. The only catch is that you’ll need to purchase the tester if you want to do it yourself. However, your trusted mechanic will likely have an automotive spark tester, so taking the car to them would be much easier and cheaper.


While not everyone has a spark tester, many people do have a multimeter at home. This is a simple device for measuring whether there’s electrical continuity in almost any electric-powered component. They’re not only suitable for testing home appliances and electrical circuits, but they also work on spark plugs and auto ignition systems.

Using a multimeter, you can test the spark plugs and ignition coils separately to determine which one has failed electrically. Whichever part has no continuity (as sensed by the multimeter) is the one that’s causing you problems.

Swapping Parts

Lastly, there’s the old-fashioned way of doing things – swapping out parts and seeing if they work.

Of course, this approach isn’t always practical. Unless you’re a ‘home mechanic’ with extra parts lying around (or have a neighbour who is one), it’s going to be tough to do.

Again, not to sound like a broken record, but your mechanic could be your best bet. If they don’t have a spark tester, they’ll have parts to swap out and test your vehicle.

Finding a Good Mechanic Near You

Finding a good mechanic near you can be a tough job but a necessity if you own a car. You can use this article or this as a guide. 

To learn more about engine misfires and other issues with your car, feel free to check out the blog at There, you’ll learn about common car problems and what you can do to solve them.

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By Ray Hasbollah

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