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Cars with an internal combustion engine all need three things in order to run. It starts with something to burn, which is an air-fuel mixture that’s pumped into the cylinder. Then it needs to be squeezed into a much smaller space, which is the compression stroke. Finally, it needs to be ignited, and that’s delivered by a spark plug in the combustion chamber. When these things happen in perfect timing across all the cylinders, the engine runs smoothly and powerfully. 

That final step of a spark plug igniting the air-fuel mixture that’s compressed in the cylinder hinges on an ignition coil delivering an electric current to the spark plug. If you have a faulty coil, it’s going to cost you between $200 and $300 on average to replace it, not to mention the frustration and downtime you’ll experience. 

Learn more about what an ignition coil is, what it does, ignition coil replacement cost, and how you can tell when it’s bad

What is an ignition coil? 

ignition coils and spark plugs on a workshop table

The main purpose for an ignition coil is to transform a small electrical charge into a pulse of high-voltage energy that ignites the air-fuel mixture compressed in a combustion chamber. The battery itself doesn’t produce enough voltage for a spark that can dependably light the air-fuel mixture since it’s only 12 volts. On average, an ignition coil will act as a transformer to elevate the spark at the spark plug to between 25,000 and 30,000 volts. If the spark is too weak, the air-fuel mixture won’t burn fully or it night not ignite at all.  

Types of ignition coils 

Early models used a conventional ignition coil that used points, then an electronic ignition coil that replaced the mechanical points with a pickup coil. A distributor-less ignition coil also offered a more dependable option and the coil could be mounted on the inner fender.  

However, today’s cars almost exclusively use a coil-on-plug design where the ignition coil fits over the spark plug itself. That means that each cylinder uses its own coil-on-plug ignition coil, so a car with a V6 engine will have six coils in use at all times.  

How does the ignition coil work? 

Although designs have changed significantly over the years, the principle of what an ignition coil does has remained the same. It uses electromagnetism to multiply an incoming electrical current to a much more powerful current when it leaves.  

It’s called an ignition coil because it contains a literal coil of wire inside. When power is supplied to the ignition coil, a magnetic field is created that amplifies the incoming power by several orders of magnitude. Then, when the current is switched off, the magnetic field collapses and that charge results in a snappy spark sent to the spark plug. 

The engine control module is responsible for timing when to cut current from each coil, resulting in an ignition sequence that precisely times your engine firing. 

How to tell when an ignition coil is bad 

There’s a chance you’ll never need to replace an ignition coil on your car, but if it is necessary, you’ll have symptoms show up. For other issues, symptoms can be intermittent but a failed coil almost always has constant problems. Signs of a bad ignition coil can include: 

  • A Check Engine Light. There will be a constant misfire in the cylinder associated with the bad coil, and the Check Engine Light will illuminate with related codes. 

  • The engine won’t run as well as it did. Effectively, you have a dead cylinder when the ignition coil isn’t working. It can feel like the engine is stumbling or about to stall. 

  • Your emissions will be higher. Because unburned fuel is being pushed into the exhaust, you may notice a raw fuel smell from the tailpipe or black smoke indicating a rich condition. 

  • Your fuel efficiency will drop. The other cylinders need to compensate for the issue, so more fuel is consumed as you drive. 

  • Your engine might not start. Especially if you have more than one bad coil or if you have an older car with a points system, electronic coil, or distributor-less coil, the engine might not fire up at all. 

You can often test coil-on-plug ignition coils quickly and easily simply with a scan tool by swapping coils from one cylinder to another and seeing if the misfire code moves along with it. 

What does it cost to replace an ignition coil? 

The cost to replace an ignition coil depends heavily on the vehicle you drive. Some common models have coils that are as little as $35 each while others can be $300 or more for a single coil. Labor costs add to the job, and it’s an important step so you correctly identify the faulty coil. The coil is almost always easy to access, so labor doesn’t add much to the ignition coil cost, though it varies by repair shop and the time it takes to diagnose the problem.  

You might want to take care of the job yourself to save on labor, and swapping a coil doesn’t take much time or any rare tools. Keep in mind that a Check Engine Light will need to be cleared using a scan tool afterward. 

Shop at AutoZone for your ignition coil parts, no matter what vehicle you’re working on. Find the right piece for the job online, or ask a AutoZone for help with part selection and advice on getting the job done. If you decide that it’s too big a job to tackle on your own, let AutoZone help you find qualified professional mechanics through our Shop Referral Program.  

FAQ/People Also Ask 

How much does it cost to change one ignition coil?

Parts can range from about $35 to more than $300 for one ignition coil, according to the vehicle you drive. Labor costs for replacing the part are usually less than $100. 

Is it worth replacing ignition coil?

Your car won’t run properly if you have a faulty ignition coil. It’s definitely recommended to replace it immediately when symptoms appear.

Can you replace just one ignition coil?

Yes. If only one coil has failed, it’s fine to change just the one. You’ll also find multi-packs if you’d prefer to change the others that are aging or you’d like to have spares on hand.

Should I replace ignition coils with spark plugs?

It’s not necessary to replace ignition coils as part of a spark plug replacement. However, if your coils are inexpensive and they’re difficult to get at, you might want to consider changing them pre-emptively.

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