Symptoms Of A Bad Ignition Coil And Replacement Cost

Do you suspect that you have a bad ignition coil? Here are all the symptoms of a bad ignition coil and what you can expect the replacement cost to be.

The ignition coil amplifies the voltage from the car battery to create an electric spark through the spark plugs.

Diesel engines do not use spark plugs as diesel combusts due to the compression in the cylinders. However, in gasoline-powered cars, ignition coils are an essential part of the engine.

Because of this, problems can occur if the ignition coil is bad, including misfires, a rough idle, and the engine becoming unable to start.

In this article, we will look at the most common symptoms, as well as other possible causes for any potential problems you are having.

Keep reading to learn everything you need to know about ignition coils, from how they work to how much it costs to replace them.

Let’s get the basics down first!

Table of ContentsShow

How Does An Ignition Coil Work?

BMW ignition coils

As previously stated, the ignition coil plays a significant role in the gasoline-powered internal combustion engine.

To be specific, it converts the relatively low voltage of the battery to the many thousands of volts that are necessary for the spark plugs to create the spark that ignites the air and fuel mixture.

The ignition coil works by an electromagnetic principle called inductance. The most important thing for you to know is that, despite the name, the ignition coil consists of not one but two coils.

There is one primary coil and one secondary. The primary coil consists of 200-300 turns of wire, and it is here that the 12 volts from the car battery are applied.

The secondary coil has many more turns of wire, around 20,000 to 30,000. This is where the amplification of volts occurs.

When the primary coil is exposed to voltage, a magnetic field is created. The magnetic field and the electric current are closely related. This means that when the current stops, the magnetic field will collapse.

As the spark plugs need to create a spark, the ignition coil stops receiving current which collapses the magnetic field. Magnetic fields can produce electricity and vice versa, so the secondary coil can produce electricity from the collapsed magnetic field.

Because the secondary coil consists of many more turns of wire, the voltage is amplified many times over, producing sufficient volts for the spark plugs to create a spark.

This process, using two coils, is called mutual conductance. The following section will give a simplified explanation but it should give you an idea of the process.

Symptoms Of A Bad Ignition Coil

auto ignition coil

Ignition coils usually turn bad due to their age but there are factors that can speed up their degradation, including bad spark plugs and heat damage.

Whatever the cause, a bad ignition coil will usually display one or more of the following symptoms:

  • Misfiring engine
  • Rough idle
  • Difficulty starting the car
  • Check engine light
  • Increased fuel consumption
  • Backfiring

These symptoms could be isolated or may appear alongside other symptoms. Each of these possible symptoms will be explained below.

Misfiring Engine

car engine with hoses

As the ignition coil is a crucial part of the ignition system, it is probably no surprise that a bad ignition coil will cause misfires.

You may notice misfires when accelerating as the car will start to jerk. This is due to inappropriate voltage generation which will negatively affect the spark plugs and the combustion.

Rough Idle

opening the hood of a broken car

For the same reasons that the engine could misfire, you might also notice a rough idle when your car is stationary. In severe cases, it can also cause your car to stall.

You might feel small vibrations when parked, but they are subtler than the misfires that happen during acceleration.

The easiest way to check this is by observing the RPM fluctuations or listening for unusual sounds coming from the engine.

Difficulty Starting The Car

man inserting key in car ignition lock

If multiple ignition coils are in bad condition, they might make the car impossible to start as the voltage is insufficient for the spark plugs to ignite.

If your car cranks but won’t start, there are many possible reasons. If the ignition coils are the culprit, there will probably be other symptoms such as misfires and rough idle before the car is unable to start at all.

Check Engine Light

car check engine light on

The infamous check engine light may illuminate when the ignition coil is bad. Unfortunately, the check engine light is usually very vague unless you have a code reader.

The check engine light might also flash while accelerating. This is often a sign of bad ignition coils or spark plugs.

Increased Fuel Consumption

how to improve gas mileage

It’s possible that the only symptom is increased fuel consumption. This happens if the ignition coil is starting to deteriorate and will soon need to be replaced.

As the ignition coils don’t work as intended, the car may compensate by injecting more fuel into the combustion chamber, impacting your gas mileage.

It can be hard to notice this symptom as not everyone keeps an eye on their fuel consumption. If you drive your car in consistent intervals and distances, it might be easier to notice this symptom.

Backfiring

black smoke coming out of exhaust

Occasionally, you might hear loud bangs with or without smoke. This is known as backfiring and is caused by unused fuel that escapes through the exhaust system.

As the faulty ignition coil does not work properly, the combustion is not completed, leading to excess fuel escaping.

Like the other symptoms listed, this is not exclusive to a defective coil ring, but it is a possible cause.

Replacement Cost

car ignition coil

If it turns out that your ignition coils are bad, your top priority should be to replace them.

As with almost any car part, the replacement cost will depend on the car’s make and model. When it comes to ignition coils it may differ even more as different cars use different types of ignition coils.

The two most common types of ignition coils are Coil-On-Plug and Coil-Near-Plug. Of the two, the Coil-On-Plug is more popular in modern cars.

If you need to buy an ignition coil, it will cost you on average $20 to $40 for a new one. There are usually offers where they are sold in a set, which will reduce the price per coil.

If your other ignition coils work fine, it is not necessary to replace the working ones. Depending on your car model, it could be more difficult to access the coil location.

If that is the case, it might be best to change all the ignition coils at once to avoid having to access them again in the near future.

In most cars, changing the ignition coils is not very labor-intensive and will cost you around $50 to $100 in labor costs.

It is not very difficult to change ignition coils. If you have experience working with cars, you can probably replace them yourself and save some money.

If you decide to do it yourself, remember to do it in a safe environment and follow the official instructions on how to replace ignition coils. Many thousands of volts pass through these coils!

Summary

Bosch ignition coils

Ignition coils play a vital role in the ignition system of gas-powered internal combustion engines.

The ignition coil converts the 12 volts from the battery to the many thousand volts needed for a spark to be created by the spark plugs. As diesel engines do not use spark plugs, they lack ignition coils.

Most problems with ignition coils overlap with symptoms of a bad spark plug. This includes misfires, rough idle, and difficulty starting the engine.

There are a few types of ignition coils, with Coil-On-Plug being the most popular in modern cars. It is relatively easy to change them yourself but you need to be careful and take all the necessary precautions.

Some car models have ignition coils that are a bit tougher to access. If that is the case, it’s probably best to change all the ignition coils at the same time.

In most cases, you will only need to replace the ignition coil that is bad, but you may get a better deal if you buy them in a pack.

The cost of a single ignition coil averages about $20 to $40. The labor cost depends on the car model but will usually set you back around $50 to $100.

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Bajro Muric

Bajro is a big car enthusiast with a love for writing and teaching. He writes about anything regarding cars - from common problems and fixes to racing.

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