What Are the Signs of Bad Ignition Coils?

Most cars on the road today have internal-combustion engines that require fuel, air, and electricity to run. On modern vehicles, the ignition coil plays an important role in supplying the electrical component to that equation. As such, if you have a faulty ignition coil, your engine will not run properly. Even worse, it may completely stall. As such, it is important to know how to identify bad ignition coil symptoms.

Ignition coils must be in good condition and performing properly for a vehicle’s engine to run correctly. As such, if your engine is not performing like it usually does, a faulty ignition coil may be why.

What Do Ignition Coils Do?

To generate power, your vehicle’s engine needs the right mixture of gasoline and oxygen. Then, it needs a spark to ignite the mixture. Car batteries only have about 12 volts. This is far shy of the several thousand volts necessary to generate the right type of spark. The ignition coil transforms the battery’s voltage to a level that allows the spark plug to do its job.

Older vehicles have a distributor, ignition coil, and spark plug wires. Most vehicles made after the late 1990s typically have a small ignition coil that sits directly on top of each spark plug, which is known as coil-on-plug (COP) design. Regardless, the functionality is the same. When you start your engine, current flows to the ignition coil. The coil, being a compact electrical transformer, then steps up the voltage level from 12 volts to as high as 60,000.

Where Is a Vehicle’s Ignition Coil?

The location of your vehicle’s ignition coil or coils depends on its make and model. Some vehicles have an ignition coil attached to every spark plug on the top of the engine block. Others have a single coil that ignites every spark plug in the engine. Furthermore, you may have a vehicle that has a cluster of ignition coils in a single pack.

Why Do Ignition Coils Fail?

Most ignition coils are durable components, but they are neither indestructible nor designed to last forever. Ignition coils tend to fail because of bad spark plugs or plug wires. If your vehicle’s fuel-to-oxygen mixture is either rich or lean, therefore, your ignition coils may fail prematurely. Additionally, engine heat and vibrations can cause damage to ignition coils. Nevertheless, on most vehicles can go 80,000 to 100,000 miles before requiring an ignition coil replacement.

Signs of a Bad Ignition Coil

1. Illuminated Check Engine Light

With most modern vehicles, a faulty ignition coil is enough to turn on the Check Engine Light. If you have an OBD-II diagnostic scanner, you may see a P0351 ignition code. Of course, because all cars are different, an ineffective ignition coil may register a different code for the problem. Alternatively, you may see a code for an engine misfire, which could be the result of a dead ignition coil.

2. Misfiring Engine

If an ignition coil is not working properly, your engine will likely misfire. A misfiring engine results in a jerking or sputtering feel when you are moving at normal speeds. When you stop, a misfire from a bad ignition coil may cause your vehicle to run roughly, vibrate or shake.

3. Hard Starts

To run, your vehicle’s engine needs a spark at the right time. If a spark plug is not getting the right voltage to create a spark, you may experience a hard start. This may mean the ignition coil is not working properly. While this problem can happen with any coil configuration, it is more common with engines that have a single ignition coil.

4. Worsening Gas Mileage

Ignition coils can either fail completely or fail intermittently. This means that you may have a coil that goes bad over time. Accordingly, even if your vehicle seems to be performing normally, you should watch for worsening gas mileage. Even if it is not caused by a bad ignition coil, decreasing gas mileage is a sure sign that something is wrong.

5. Diminished Power

Vehicles with bad ignition coils often run roughly. This means, when your vehicle is idling, you may notice a rough feel or sound. Your vehicle may also experience a drop in power. As a result, a bad ignition coil can make it acceleration more sluggish. The vehicle may even stall.

6. Sudden Backfires

Modern engines should not backfire. This loud bang happens when unburned fuel suddenly escapes out of the exhaust system. While there are a few causes of backfires, a failing ignition coil is a common one. Regrettably, if you do not address backfires quickly, they can lead to significant engine damage.

How Do You Replace a Damaged Ignition Coil?

Many ignition coils offer a plug-and-play design, making them easy to replace. If your vehicle has this style of coils, even a relatively new DIYer can likely handle the replacement without professional assistance. Still, some coils are hard to access. If you do the job on your own, though, you only pay the parts costs. While the price of replacement ignition coils varies widely, on most vehicles, it ranges from $40 to $300 for parts. If you have a mechanic do the job, you must pay labor costs. These likely depend on the design of your engine and the location of the bad ignition coil.

The mechanic will be able to clear fault codes after the repair. If you do not have an OBD-II tool and still want to do the job yourself, go to AutoZone to use a Loan-A-Tool® scanner.

What Is Your Next Step?

To keep your vehicle running optimally, you need ignition coils that function properly. Watching for bad ignition coil symptoms is critical. Also, replacing your ignition coils per your vehicle’s manufacturer’s instructions may help you avoid car trouble.

Whether you need ignition coils, tools, or anything else car-related, AutoZone has your back. For top-grade components, complete your order online or visit your neighborhood AutoZone.

Advice, how-to guides, and car care information featured on and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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