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.

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

Because of that, many problems may occur if the ignition coil is bad, including misfires, rough idle, and the refusal of the engine to start.

In this article, the most common symptoms will be described as well as other causes that may cause any potential problems you may have.

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

Let’s get started with the basics.

Table of ContentsShow

How Does An Ignition Coil Work?

BMW ignition coils

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

More precisely, it converts the relatively low voltage from the battery to many thousands of volts which is needed for the spark plugs to create a spark and consequently ignite the air and fuel mixture.

The ignition coil work by an electromagnetic principle called inductance. To put it in simple terms, the ignition coil consists of not one, but two coils.

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

The secondary coil has many more turns of wire, around 20.000 to 30.000. This is where the amplification of volts happens.

When the primary coil is exposed to voltage, a magnetic field will be created. The magnetic field and the electric current are tightly 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, the secondary coil can therefore produce electricity from the collapsed magnetic field.

Because the secondary coil consists of many more turns of wire, the voltage gets amplified many folds, producing a sufficient amount of volts for the spark plugs to create a spark.

This phenomenon, with two coils, is called mutual conductance. This is more of a simplified explanation but will hopefully give you an idea of the process.

Symptoms Of A Bad Ignition Coil

auto ignition coil

Ignition coils usually get bad due to their age but there are factors that may speed up their degradation, including bad spark plugs or heat damage.

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

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

These symptoms may either be isolated or may present with multiple symptoms. Each of these symptoms will be discussed below.

Misfiring Engine

car engine with hoses

As the ignition coil is a crucial part of the ignition system, it should be 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 mess with the spark plugs and the combustion.

Rough Idle

opening the hood of a broken car

For the same reasons the engine might misfire, you may notice a rough idle when standing still. In severe cases, it can also cause your car to stall.

Even here, you might feel small vibrations when parked, they are more subtle than misfires that happen during accelerations.

The most obvious sign is by observing the RPM’s fluctuating abnormally or hearing the engine make a strange irregular sound.

Difficulty Starting The Car

man inserting key in car ignition lock

If multiple ignition coils are in a bad state, it might make the car impossible to start as the voltage is not sufficient for the spark plugs to ignite.

If your car cranks but won’t start, there are many potential reasons. If the ignition coils are the culprit, other symptoms such as misfires and rough idle are usually present prior to not being able 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. Unfortunate, in most cases, the check engine light is highly unspecific unless you have a code reader.

The check engine light might also flash while accelerating, this may be a sign of bad ignition coils or spark plugs.

Increased Fuel Consumption

how to improve gas mileage

It is possible that the only symptom is increased fuel consumption. This happens if the ignition coil is starting to get bad and is counting its last days.

As the ignition coils don’t work as they should, the car may compensate by injecting more fuel into the combustion chamber, increasing the gas mileage.

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


black smoke coming out of exhaust

Occasionally, you may hear loud bangs with or without smoke. This phenomenon is called 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 complete leading to unused fuel escape.

Just like other symptoms listed, this one is not exclusive to a defective coil ring, but the possibility is there.

Replacement Cost

car ignition coil

If it turns out that your ignition coils are bad, the next thing you should do is to replace them.

As with almost any car part, the replacement cost depends on the car 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. Out of these two, the Coil-On-Plug is more popular among modern cars.

To buy an ignition coil, it will cost you on average $20 – $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 needed to replace the other coils. Depending on your car model, it might be a bit more difficult to access the area.

If that is the case, it might be wise to change all the ignition coils if they are close to their expected life span.

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

It is not very difficult to change ignition coils. If you are handy, you may replace them yourself and save a few bucks.

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


Bosch ignition coils

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

The ignition coil converts the 12 volts from the battery to 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, Coil-On-Plug being the most popular in modern cars. It is relatively easy to change them yourself but be careful and take all precautions necessary.

Some car models might have ignition coils that are a bit tougher to get by, if that is the case, it may be good to change all the ignition coils in the hard-to-reach spot.

Generally, you only need to replace the ignition coil that is bad, but you may get a bigger price if you buy them in packs.

The cost of one ignition coil averages about $20 – $40. The labor cost depends on the car model and will 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.