Ignition Coils Help Spark Horsepower

DIY

Engines & Components

Chris Tonn

February 13, 2026

Chris Tonn

February 13, 2026

Every gasoline-powered vehicle relies on a spark to get things going. As part of the four-stroke combustion cycle, a spark plug - opens in new window or tab. creates a spark to ignite a fuel-air mixture.

The spark comes from several hundred volts of electricity jumping across electrodes, but the battery - opens in new window or tab. in the vehicle only produces around 12 to 14 volts. The extra juice comes from an ignition coil - opens in new window or tab..

An ignition coil is an electrical device that converts the low voltage from the battery to the high voltage needed to create the spark. It’s a critical link in the chain to start and run any gasoline-powered engine, and a failing coil can keep a car from starting, cause it to stall, or cause the engine to misfire. Sometimes, the misfire is so severe that the vehicle will shake while driving.

What Is an Ignition Coil?

An ignition coil works on the elementary electrical principle of magnetism. Various gauges of copper wire wind around an iron core. As current from the battery - opens in new window or tab. flows into the winding, it builds up in the heavier-gauge primary coil windings. When these windings become charged, a discharge causes an oscillation within the magnetic field of the smaller secondary windings.

This oscillation creates very high voltage, which travels to the spark plug. This charge and discharge cycle happens hundreds or thousands of times per minute, depending on the engine and how fast it’s running.

Shop now for ignition coils - opens in new window or tab.

Many older cars have a single ignition coil that works in concert with a distributor - opens in new window or tab.. The distributor is connected to the engine and spins at the same speed as the engine. The distributor takes the incoming high voltage from the ignition coil and distributes it via spark plug wires - opens in new window or tab. to the spark plugs, sending a spark at the exact time each cylinder needs it to continue running.

This distributor system has many moving parts that can fail over time and needs a good bit of maintenance to run efficiently. Over the past 30 years or so, nearly all cars on the market have switched to a distributorless ignition system. But without the distributor, how does the voltage get high enough to create a spark?

Read: How a Car Engine Works - opens in new window or tab.

An EMPI ignition kit for Type 1 VW engines with a distributor, ignition coil, and spark plug wires

Coil-on-Plug Ignition Systems

Most cars these days have individual coils on each spark plug. Instead of spark plug wires snaking through an engine compartment, a small electrical component is stacked atop each cylinder and connected directly to each spark plug. These components are ignition coils just like the larger coil used on older cars, but instead of connecting to a distributor, they communicate with the engine control unit - opens in new window or tab. to fire the spark plug as required.

Four coil-on-plug or pencil style ignition coils

These coil-on-plug distributorless systems have proven much better for modern engines - opens in new window or tab., with the ability to react quickly to the numerous sensors throughout the vehicle and to adapt to varying efficiency and performance requirements. They are quite robust, too, as they have fewer moving parts than a traditional distributor, which can wear down over time.

However, that’s not to say they can’t fail. Vibration, heat, and other environmental factors within an engine compartment can lead to the demise of any electrical component.

Shop now for ignition coils - opens in new window or tab.

Symptoms of a Failing Ignition Coil

Once you turn the key or press the start button on your car, you don’t usually think about the engine as it hums away. But if you notice a hiccup in how smoothly your engine runs, it can be a sign of problems. A failing or dead ignition coil could cause any of these symptoms:

Engine not starting or hard to start
Engine misfiring or backfiring
Poor fuel economy
Engine shaking while driving

Many of these symptoms occur when a coil fails because little or no voltage is getting to a single spark plug. In this case, the rest of the cylinders will work normally, but the cylinder without a spark doesn’t burn the fuel and air mixture within. This causes a stumble in the combustion cycle. Since the engine is turning thousands of times a minute, this stumble can cause the engine to shake as you drive.

How to Test an Ignition Coil

It’s somewhat easier to diagnose a failed ignition coil on cars built after 1996, as an engine misfire will typically register a check engine light that can be diagnosed with an OBDII code reader - opens in new window or tab.. The code reader usually won’t give you an exact answer, but it may read something like “cylinder two misfire,” which will tell you to look at cylinder two for a problem.

An ignition kit for a Chevrolet V-8 engine with spark plug wires, spark plugs, ignition coils and hardware — An ignition coil, spark plug wire, and spark plug kit for a Chevrolet V-8

For older cars, a misfire can sometimes be diagnosed with an infrared thermometer - opens in new window or tab.. Point the thermometer at each exhaust port as the car is running. A misfiring cylinder will usually be significantly cooler than the properly firing ones.

If you are working with an older car using a single coil and a distributor, a misfire is usually not caused by a failing coil.
A failing coil on these older engines usually means the car won’t start or will run poorly on all cylinders.
A misfire on a distributor-equipped engine is usually the result of timing issues within the distributor or failures of the spark plug wires.

Shop now for all ignition system parts - opens in new window or tab.

For modern coil-on-plug engines, an easy way to diagnose a failed ignition coil is to swap coils (and their associated clip-in electrical connections - opens in new window or tab.) with a known good coil from an adjacent cylinder. If the problem follows the coil to the new cylinder—which you can tell from your OBDII code reader—then you’ve traced the problem to the coil. If the previous cylinder remains dead with a good coil, then there is likely a problem with the spark plug.

The cost to replace an ignition coil can vary greatly depending on the type of car and coil. For traditional single-coil ignition systems, a coil usually costs between $75 and $200. The replacement job often takes an hour or two, so figure around $150 to $300 in labor charges. A coil-on-plug ignition coil may cost more, up to $500. However, the labor could be less expensive because these coils are easier to access.

Read: Spark Plug Replacement Step-by-Step - opens in new window or tab.

About the author

Chris Tonn

Some enthusiasts say they were born with gasoline in their veins. Chris Tonn, on the other hand, had rust flakes in his eyes nearly since birth. Living in salty Ohio and being hopelessly addicted to vintage British and Japanese steel will do that to you. His work has appeared in Hagerty, The Truth About Cars, Reader's Digest, AutoGuide, Family Handyman, and Jalopnik. He's currently looking for the safety glasses he just set down somewhere.

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This article is meant to provide general guidance only. Automotive maintenance, repair, upgrade, and installation may depend on vehicle-specifics such as make and model. Always consult your owner's manual, repair guide for specific information for your particular vehicle and consider a licensed auto-care professional's help as well, particularly for advance repairs.