What Is an Ignition Coil?

By: Talon Homer  | 
ignition coil
A modern ignition coil just looks like a rubber boot that slips onto its spark plug. BLKstudio/Shutterstock

The ignition coil can be found on practically every gasoline-powered internal combustion engine vehicle on the market. In fact, your car probably has several of them — one for each engine cylinder. The ignition coil uses electrical induction to supply your engine with the high voltages necessary for proper fuel combustion. That sounds like a simple task, but it's important when it comes to your car's fuel efficiency and reliability. Like most major components, these coils should be inspected and maintained regularly.

Gasoline vehicles are usually powered by a simple 12-volt DC battery. This is enough voltage to engage a starter motor and get the engine turning, but it's not enough for actual combustion. (A spark plug needs around 40,000 volts for combustion.) That's where the ignition coil comes in. It contains a step-up transformer (also known as a coil pack) to convert that tiny current into quick bursts of several thousand volts at a time. This electricity is then sent to each spark plug in perfect time with the cylinder's combustion cycle, igniting fuel vapor with oxygen and producing power. Diesel engines notably lack ignition coils because they ignite their fuel using heat and pressure alone.


A modern ignition coil just looks like a rubber boot that slips onto its spark plug. Up top, there's an electrical connector for receiving voltage and some threads to mount it securely on the engine. If you were to dissect the component, however, you would find several meters of tightly wound copper wiring that make up the coil pack. These are wrapped around an iron core which receives the 12V current to serve as an electromagnet. The energy stored in the magnetic field of the core is the energy that is transferred to the spark plug. Lastly, the whole coil pack is embedded in a resin insulator to prevent short-circuiting. The ignition coil uses a principle known as Faraday's law of induction to produce incredible amounts of power, and the same law applies when constructing electric motors as well as generators.

Early cars came with one ignition coil. This coil pack was contained in a part called the distributor which was essentially a bundle of wires leading to each spark plug. The distributor was also responsible for timing each fuel ignition and could become de-calibrated over time, causing the engine to run out of sync. Around the 1980s, automakers began installing electronic sensors to handle ignition timing, and multiple ignition coils for better power and efficiency. Nowadays, most engines have an ignition coil placed directly on top of each spark plug. This is known as "coil on plug" configuration.

These multiple-coil-pack-systems are much more reliable than the distributors of the past, but individual ignition coils can still fail over time and lead to an engine misfire condition. When a cylinder misfires, that means the fuel inside it is not burning at the correct time or not burning at all. This is also sometimes referred to as a dead cylinder. Prolonged misfiring can lead to larger problems like warped piston rods or corrosion inside the cylinder wall. It can also cause unburned fuel to leak into your exhaust system, which is bad for the environment and worse for your car's catalytic converters. Once identified, a misfire should be addressed as soon as possible.


Diagnosing a Bad Ignition Coil

Ignition coils typically have a long life of operation — over 100,000 miles. However, they can malfunction eventually, causing that dreaded misfire. This issue will probably start out slowly while being hardly noticeable but will become more problematic over time. When a misfire occurs, even for a few seconds, your engine computer management will likely detect the issue and generate a trouble code. At which point the check engine light comes on.

With a diagnostic scanner at a garage or auto parts store, you can then see which cylinder is misfiring and proceed from there. Another sign of a misfire can be a rough engine idle. In extreme cases, the vehicle may feel like it's surging at a high RPM or like it may stall at low speeds.


Other signs that you have a bad ignition coil are worsening gas mileage, sluggish acceleration and a backfiring engine, meaning you hear a loud bang as unburned fuel escapes from the exhaust system.

Apart from the coils, another likely culprit for misfiring is the neighboring spark plug, faulty spark plug wires or a larger electrical issue. When you take your car to a professional shop, they have an array of voltage testing equipment to see where the malfunction is happening along the circuit. They may also look for other things like clogged fuel injectors.


Ignition Coil Replacement Cost

fastening the bolt of car ignition coil
An auto mechanic fastens the bolt of the car ignition coil with a socket wrench to fix the vehicle ignition system. BLKstudio/Shutterstock

Once you've identified a faulty ignition coil, replacing them usually costs about $250 per part. Then there's labor on top of that, which can easily add an extra $50 or more per coil. If you're handy, this is a job you may want to attempt yourself since the ignition coils are usually located in an accessible spot on top of the engine and can be removed with a basic socket wrench. Just detach the 12V battery first to be safe.

At the same time, you may want to consider replacing all of your car's spark plugs. The plug in the misfiring cylinder could've become corroded or caked with residue, and it's probably due for a replacement anyway. Spark plugs will generally be harder to reach than ignition coils and require specialized sockets to unbolt from the combustion chamber. If all that sounds too complicated, your best bet will be to seek a professional mechanic.


With multiple ignition coil and spark plug replacement jobs, a shop can easily rack up thousands of dollars of parts and labor on your bill. It's also likely to put your vehicle out of commission for at least a few hours. If your car is still subject to a manufacturer's powertrain warranty, then that can save you a bunch of money while getting maintenance like ignition coil replacement done at an approved garage.

Ignition Coil Auto FAQ

How much do ignition coils cost?
The cost is about $250, plus labor costs.
How do you test an ignition coil?
Use a diagnostic scanner, available at an auto parts store or ask a mechanic to do it.
What is an ignition coil?
Your car engine has a coil, which is basically a high-voltage transformer composed of two wire coils. One of the coils is referred to as the primary coil, and the secondary coil wraps around it.
How long do ignition coils last?
They are supposed to last at least 100,000 miles.
What are the symptoms of a bad ignition coil?
You'll get a "check engine" light coming on. Your car may also feel like the car is surging at a high RPM or like it may stall at low speeds.