What Is an Ignition Coil?

If you regularly hop inside your gasoline-powered vehicle, twist the key (or press the start button), and are greeted with the familiar roar of the engine coming to life, you can thank the ignition system—more specifically, the ignition coils. These essential components help to ignite the air-fuel mixture in the cylinders, leading to a series of small explosions (called combustion) that generate power. 

In this article, we’re breaking down everything you need to know about ignition coils, including what they are, what they do, how they work, where they’re located, and what happens when they fail. 

Along the way, learn more about common ignition coil types, symptoms of bad coils, how to test them, and the process of replacing faulty units. Whether you’re troubleshooting an intermittent misfire or want to better understand your ride, this guide has you covered. 

An ignition coil performs a critical yet straightforward function in a vehicle’s ignition system. This crucial component transforms a vehicle’s low battery voltage into the high voltage “spark” necessary to ignite the air-fuel mixture within the engine’s cylinders. 

Spark plugs need approximately 20,000 to 40,000 volts of electricity to create a strong enough spark. However, a vehicle’s battery and charging system only produce around 12, so where does the extra juice come from? 

Ignition coils act like high-voltage transformers, boosting the voltage and feeding it to the spark plugs at the precise time to keep the engine firing smoothly and efficiently. 

An ignition coil is a power booster that operates on the principle of electromagnetic induction. As the engine runs, the ignition coils store electrical energy from the charging system. At the exact moment each cylinder needs a spark, the coil releases and multiplies its stored energy, resulting in a momentary discharge of 30,000+ volts. This high-voltage burst travels directly to the spark plug, which ignites the air-fuel mixture in the cylinder. 

On most modern vehicles, ignition coils are mounted directly to the valve covers, above the spark plugs. However, some older vehicles use remote-mounted ignition coils near the engine or firewall that send power either directly to the individual cylinders or to a distributor. Regardless of the design, ignition coils are typically mounted in proximity to the cylinders for efficient power delivery. 

For the most part, over 90 percent of the gas-powered vehicles you’ll see on the road today feature one coil pack per cylinder, which is typically referred to as a coil-on-plug (COP) setup. However, configurations can vary. 

For instance, some older vehicles feature a single ignition coil with wires running to each cylinder, while other high-performance applications may use two coils per cylinder, each operating two plugs. 

Other applications, such as diesels or electric vehicles, don’t use ignition coils. 

There are several types of ignition coils, with some of the most popular being: 

• Distributor-Coil: A system implementing a single coil that feeds spark through a distributor, which times the energy’s release to individual cylinders through spark plug wires. 

• Waste Spark Coil: A single coil that fires two cylinders simultaneously. 

• Coil-on-Plug (COP): Individual coils mounted directly onto each spark plug. 

• Coil Packs: A grouped set of coils serving multiple cylinders. 

Ignition coils are designed to last tens—if not hundreds—of thousands of miles. However, like all electrical components, they aren’t impervious to failure. As vehicles age and rack up miles, the countless heat cycles and hours of vibrations can take their toll on coils, leading to failure. 

Understanding how and why ignition coils fail can help you diagnose problems and prevent more disastrous engine issues down the road. 

Ignition coils can fail over time—however, failures don’t often occur for hundreds of thousands of miles. When failures do occur, they’re typically due to age or environmental conditions rather than design flaws. 

When an ignition coil begins to fail or quits completely, you’ll know. Driveability issues, like misfires, are the most common symptom, as the coil can no longer deliver the current required to ignite the spark plug. In most cases, a misfire will feel like a rhythmic stumbling of the engine, hesitation, or a rough idle. 

If left unaddressed for an extended period, a failed ignition coil can damage spark plugs, catalytic converters, and more. 

Ignition coil failures are rare, but they can occur for several reasons. Some of the most common causes include: 

• Exposure to excessive heat

• Continuous vibrations

• Worn or incorrectly gapped spark plugs

• Oil or moisture (coolant) contamination

• Electrical spikes

• Poor ground connections or damaged wiring harnesses

A failing ignition coil is obvious, so long as you know what to look for. The damaged component will manifest in various ways, including performance issues, check engine lights, and unpleasant sounds. If you suspect your vehicle has one or more bad ignition coils, look out for one or more of the following symptoms: 

• Engine misfires

• Rough idle

• Hesitation while accelerating

• Stumbling or surging

• Loss of power

• Poor fuel economy

• Difficulty starting

• Engine stalling

• Flashing check engine light

• Strong fuel odor from the tailpipe

Because symptoms of a bad ignition coil can overlap with other fuel and ignition system issues, it’s essential to verify whether or not your problem lies in the ignition coil before replacing parts. Depending on your access to tools and mechanical knowledge, consider testing your vehicle’s ignition coils with the following methods: 

• Use an OBDII scanner to check the computer for active and stored ignition coil codes, which typically present as misfire codes. These codes always begin with “p030,” with a fourth digit indicating the cylinder with the misfire. For example: 

  • P0300: Multiple/random cylinder misfire

  • P0301: Cylinder 1 misfire detected

  • P0302: Cylinder 2 misfire detected

  • P0303: Cylinder 3 misfire detected

  • P0304: Cylinder 4 misfire detected

  • P0305: Cylinder 5 misfire detected

  • P0306: Cylinder 6 misfire detected

  • P0307: Cylinder 7 misfire detected

  • P0308: Cylinder 8 misfire detected

• Swap the suspected ignition coil with one from another cylinder. Clear the codes and restart the vehicle. If the misfire code transfers to another cylinder, you’ve found your issue. 

• Unplug the suspected ignition coil while the engine is running, listening closely for any change in the engine's tone and RPM. There should be a notable RPM drop when a functioning coil is unplugged. However, unplugging the faulty coil shouldn’t cause a difference. 

• Inspect the ignition coil and surrounding wiring for visible damage. If you see burnt marks, corrosion, oil contamination, or damaged coil boots, it’s likely time for a new ignition coil. 

• Perform a spark test using a spark tester. This tool installs inline between the coil pack and spark plug. If the coil is producing proper voltage, the bulb in the spark tester will illuminate. 

Curious how often you should expect to replace your vehicle’s ignition coils, or how much a replacement will cost? Let’s find out. 

Unlike spark plugs or other wear components, ignition coils typically don’t need to be replaced on a fixed schedule. They aren’t a maintenance part—instead, they should only be replaced when they show signs of wear or failure. 

That said, replacing these components as part of a tune-up isn’t a bad idea on a high-mileage vehicle. Parts typically aren’t that expensive, and replacement isn’t often more difficult than removing a single bolt and unplugging a single electrical connector. 

Ignition coils aren't typically a pricey part, though the cost of replacement can vary significantly based on the number of coils being replaced, whether the mechanic is also replacing other components, like spark plugs or valve cover/tube seal gaskets, and the difficulty/time of the repair.  

For example, a mechanic may charge as little as $150 to $300 to replace a single coil pack on a vehicle with easy-to-access ignition coils, good condition spark plugs, and no oil in the spark plug wells. 

However, this price can skyrocket if the mechanic finds multiple faulty ignition coils, fouled spark plugs, and leaking valve cover gaskets. Expect to pay well over $1,000 for repairs such as these. 

Driving with a bad ignition coil, for any distance, isn’t recommended. While you may be able to drive short distances, prolonged driving can result in engine damage, poor fuel economy, and catalytic converter failure. If the symptoms are severe enough, your vehicle may become inoperable. 

Taking proper care of your vehicle’s ignition system helps to ensure reliable performance, better fuel efficiency, and longer service life. Follow these tips to keep your ride’s ignition system operating efficiently and your ride running smoothly.

• Replace the spark plugs at recommended intervals. Use only high-quality plugs, and gap the electrodes to the manufacturer's recommended specifications. 

• Fix oil leaks that could contaminate the coil packs, like leaking valve cover gaskets and spark plug well seals. 

• Avoid cleaning the engine bay with high-pressure water, as it can introduce moisture to electrical components. 

• Address check engine lights, especially those that flash rapidly, quickly to reduce the chance of catastrophic engine damage. 

• When replacing coil packs, use only high-quality, OEM-grade parts. It’s better to spend a little extra money on premium parts than save money on cheap components that’ll likely fail again.