Your car’s ignition coil is an essential component in the engine’s ignition system, transforming the battery’s voltage into the energy required to create the spark that ignites the fuel-air mixture in the engine.
When an ignition coil fails, it can cause several issues, affecting engine performance and fuel efficiency. Recognizing the symptoms of a bad ignition coil can help you address the problem early, avoiding more serious and costly repairs.
In this article, we’ll cover common signs of a bad ignition coil, the role it plays in your engine, and tips on diagnosing and addressing ignition coil issues.
What Is an Ignition Coil?
An ignition coil transforms your car’s low-voltage battery power (12V) into the high-voltage spark (20,000-50,000V) needed to ignite the fuel-air mixture in the engine cylinders.
Modern vehicles often use coil-on-plug (COP) systems, with one coil per cylinder, while older models may have a single coil or coil packs serving multiple cylinders.
A failing ignition coil disrupts this process, causing incomplete combustion and performance issues.
This problem is common in high-mileage cars (over 100,000 miles), especially in brands like Ford, Toyota, Honda, and Chevy. Ignoring it can lead to damaged catalytic converters, failed emissions tests, or even engine failure.
What Does an Ignition Coil Do?
An ignition coil is a transformer that converts battery voltage into the tens of thousands of volts needed to fire the spark plugs.
Ignition coil operation is based on a phenomenon called electromagnetic induction—a process that uses a magnetic field to generate electrical current.
Inside the coil, there are two sets of windings. The so-called primary windings contain a hundred or so coils of wire, whereas the secondary windings have thousands of coils of wire.
When the ignition coil is energized, battery voltage flows through the primary windings, creating a magnetic field.
Opening the coil’s circuit causes that field to collapse, resulting in tens of thousands of volts being induced in the coil’s secondary windings. The high voltage from the secondary windings then travels from the coil to the rest of the ignition system.
It’s important to note that there are different types of ignition coils. Older vehicles have a single ignition coil that’s connected to a distributor via an ignition wire.
High-voltage travels from the coil to the distributor, which, in turn, distributes that voltage to the individual spark plugs and wires.
Newer vehicles use either coil-on-plug (COP) ignition coils or coil packs. Engines with a COP ignition system have one ignition coil per cylinder. Each coil sits directly on top of a spark plug.
Meanwhile, a coil pack is a group of ignition coils combined together in a single molded block. With this design, each cylinder gets its own ignition tower but shares its coil with a companion cylinder.
Symptoms of a Bad Ignition Coil
The most common symptoms of a bad ignition coil include the check engine light appearing on the dashboard, along with engine misfires, stalling, and rough idling.
You might also notice poor acceleration, reduced fuel efficiency, hard starting, or exhaust fumes that smell like unburned fuel.
Here’s a closer look at the most common signs of a bad ignition coil and how they affect your car’s performance.
#1. Check Engine Light.
Bad ignition coils usually cause the Check Engine Light to come on. If you use your compatible code scanner, you may see a P0300 – P0312 ignition code or something manufacturer-specific. You may also get a generic engine misfire code.
If the Check Engine Light is solid, you may have time to repair the problem before it becomes severe. However, further damage can occur if you keep driving. If the light is flashing, you should turn off the car immediately to protect the engine from damage.
#2. Misfiring Engine.
When the ignition coil doesn’t work correctly, the engine is going to misfire. A misfiring engine occurs because the air-fuel mixture in the cylinder can’t ignite.
Misfiring engines create a hesitation or jerking feeling, especially as you attempt to accelerate. You can also feel the vibration throughout the cabin as the misfire gets worse.
#3. Stalling Engine.
If the bad ignition coil is unable to keep the engine running, it’s going to stall. This happens when combustion doesn’t occur as it should.
In severe conditions, you won’t be able to restart the car engine. This situation leaves you stuck wherever you are when the engine stalls.
#4. Rough Idle.
Engine idling defines the time when the motor is running, but the car isn’t going anywhere. You leave the engine idle when you are stopped at a traffic light or intersection. You can also leave the engine idling when you are parked and waiting.
In general, the engine idle should be smooth and predictable. If it becomes rough, there could be an issue with the ignition coils. It can also indicate a problem with the fuel injectors, spark plugs, air filter or exhaust.
#5. Rough Acceleration.
The rough-running engine won’t only occur while idling. It can be felt when you are driving as well. As you push down the accelerator, you might notice the engine struggling more than normal.
As the rough ride gets worse, the cabin will start to vibrate. Eventually, you will start to worry about the engine stalling.
#6. Poor Fuel Economy.
An ignition coil can fail spontaneously or over time. As the coil goes bad over time, you should notice a decrease in fuel economy.
In fact, if you are watching the gas mileage, you should notice problems before other symptoms appear.
You want the engine to run as efficiently as possible and you don’t want to spend more money on fuel than needed, so it’s best to replace the bad ignition coil as soon as possible.
#7. Hard-Starting Engine.
To start the engine, there must be a spark igniting the air-fuel mixture at the right time. If the spark plug isn’t receiving the proper amount of voltage from the ignition coil, the mixture won’t ignite properly.
This problem is often seen most often when trying to start the engine. If you have trouble getting the engine started, consider that the ignition coil or other components in the system could be the fault.
#8. Exhaust Smells Like Gas.
The ignition system is responsible for igniting the air-fuel mixture in each cylinder. If it doesn’t, there’s going to be an imbalance of fuel and air going through the engine.
The result is an excess of gas coming out of the exhaust. Fuel has a distinct smell, so you are going to notice the unburned gas as it exits.
What Causes Ignition Coil Failure?
Like any part, ignition coils are subject to wear and tear over time. For many vehicles, it’s a good idea to consider ignition coil replacement around every 80,000-100,000 miles or so. However, other circumstances may lead to premature ignition coil failure, like:
#1. Poor Air-Fuel Mixture Ratio.
If the air-fuel mixture isn’t at the correct ratio, your engine is likely to run too lean (too much air) or too rich (too much fuel). When either happens, your ignition coils are forced to work overtime to deliver power to the spark plugs.
- Lean mixtures require higher voltage to ignite, which can overwork and overheat ignition coils.
- Rich mixtures leave behind carbon buildup, which can foul spark plugs and interfere with the coil’s ability to send power effectively.
#2. Spark Plug Gap Too Big.
For spark plugs to operate properly, the space between their electrodes must be gapped correctly. Spark plug gaps naturally widen over time, so it’s important to monitor their health.
Too small of a gap, and the spark generated may be too weak to efficiently combust fuel. Too large of a gap, and the spark may not jump the gap consistently.
To bridge a larger gap, ignition coils must deliver more voltage than otherwise necessary, leading to faster wear out.
#3. Valve Cover Leaks.
While today’s coil-on-plug setups deliver stronger and more accurate sparks than single coil systems of the past, their position under the hood makes them more susceptible to moisture—and it’s no secret that electronics and moisture don’t mix well.
In many COP systems, the coil sits directly above its spark plug, which resides inside a tube in the engine’s valve cover.
Over time, the rubber between the spark plug tube and the valve cover may break down and let oil inside of that tube, shorting the spark from ignition coils and fouling spark plugs in the process.
#4. Excessive Engine Heat.
High temperatures under the hood can cause ignition coils to overheat and degrade faster. Excessive engine heat can stem from various issues, including cooling system problems like low coolant, a stuck thermostat, or a clogged radiator.
#5. Electrical Overload or Faulty Wiring.
Bad wiring or poor grounds can damage ignition coils.
Even a weak battery or malfunctioning alternator can affect coil performance by delivering inconsistent power.
Diagnosing Ignition Coil Problems
When your car isn’t running right, it’s key to determine if the ignition coil is to blame before replacing it. To diagnose the issue, here are some tests you can perform:
- Scan the Check Engine Light. Common DTCs you’ll encounter when there’s a bad coil include P0300 through P0308 for misfires, P0350 through P0362 for ignition coil circuit malfunctions, and P2300 though P2311 for specific ignition coil faults.
- Check for spark. Remove a spark plug, connect it to the coil, then crank the engine to see if a strong blue spark is produced. If not, there’s a problem.
- Measure the coil resistance with a multimeter, comparing it to the specs in the repair manual. Out-of-range readings suggest an issue.
- Swap the coil. Switch the coil between cylinders, then check if the fault codes follow the coil. If they do, it indicates it’s a bad coil.
- Visual inspection. You can sometimes see burn marks, cracks, or corrosion on the coil.
How to Change Ignition Coils?
Running with a bad ignition coil is damaging to your car. Follow the following steps to replace your malfunctioning coil with a new one.
#1. Take safety precautions.
Make sure your car has cooled down before opening the hood. Assuming that you have read and understood your vehicle’s user manual, go ahead and disconnect the negative terminal of the battery before you touch anything else.
Please note that some cars recommend that you use a memory retainer if you intend to keep your car dismantled for a long time.
#2. Remove the coils.
The most difficult part about removing ignition coils is you have to go through obstacles like the sparkplug and coil insulator boots to get to them.
If yours is the kind of car that you have to remove the plenum manifold to access the ignition coils, then you need to have a new gasket.
Carefully remove the ignition coil bolts without damaging the electrical plastic connector. Twist gently to slide and remove the insulator boot from the sparkplug.
Perform all these actions with care because if the insulator boot tears and a piece remain at the sparkplug, it will cause a misfire the next time you start the car.
Remove one coil at a time and identify each with a picture, number mark, or coil pigtail so that you don’t confuse them during the assessment.
Thoroughly check for signs of oil or any contamination in each coil insulator boot and fix the leaks before replacing the ignition coils.
#3. Test Each Ignition Coil.
You need to test the ignition coils because sometimes when the check engine light flashes, it only reports a misfire. Something else apart from the ignition coils might have caused the misfire. Look for a code reader or find an auto shop to find what the code symbolized.
The code will relate the misfire to the cylinder with the problem, and from there, you can identify the ignition-related part that caused the misfire.
If you removed multiple coils and you identified the ignition coil side with the misfire, try switching the coils to see if the malfunction follows the suspected coil to the other side.
If it does, then you can proceed to step number 4. That is also one way how to tell if the ignition coil is bad.
If the misfire doesn’t follow the coil, test both coils using a multimeter as explained earlier to identify the faults.
#4. Install the New Coils.
Lubricate your new ignition coils before you install them. Confirm that you have the right oil. Also, double-check the terminal pins and pigtail connections.
Apply dielectric grease to the insulator boot of the new ignition coil to provide a barrier against moisture. The grease will also aid to reduce friction the next time you need to remove the coil.
Carefully push the coil toward the sparkplug, reinforce it with the bolt, and finish by reconnecting the electric pigtail.
You can now return the components you removed and test if your ignition coil replacement is successful by taking your car for a test drive.
Ignition Coil Replacement Cost
On most modern vehicles, ignition coil replacement is a relatively straightforward repair, but prices vary based on parts cost and accessibility.
At an independent repair shop, the total cost usually falls between $200 and $450 per coil, including parts and labor. Labor typically runs $150 to $300, which reflects about one to two hours of work depending on how easy the coils are to access.
Dealerships charge more, often $400 to $600 or higher per coil, especially on luxury or performance vehicles. Basic four‑cylinder cars are usually on the lower end, while V6, V8, and luxury vehicles tend to be more expensive.
If you replace the coil yourself, expect to pay $50 to $150 per coil for the part. For many vehicles, coil replacement requires only basic tools, making it a manageable DIY job.
FAQs.
Q: How long do ignition coils last?
Ideally, ignition coils should last around 100,000 miles. However, several factors can reduce that mileage.
Ignition coils work symbiotically with other engine parts like sparkplugs. When one part malfunctions, it overworks the other. Therefore, the lifespan of ignition coils is determined by the overall condition of the engine.
Q: Can an ignition coil get weak?
Yes. Ignition coils can get weak from vibrations due to the vehicle’s motion or heat conditions. Heat and vibrations open the primary and secondary windings of ignition coils, reducing their efficiency.
Another critical thing that can weaken ignition coils is the high resistance of the spark plugs. Sparkplug resistance causes the ignition coils to get very hot and makes them burn through the insulation causing electrical shorts.
Q: Should you replace ignition coils with sparkplugs?
Most of the time sparkplugs and ignition coils get damaged together or are the reason for the damage. Always test the sparkplugs whenever you are replacing ignition coils.
Although it isn’t always necessary to replace sparkplugs together with ignition coils, it is pointless to install new ignition coils and leave the sparkplugs that caused the coils to malfunction.
Q: What color is a weak spark?
A yellow or red spark is a weak spark that symbolizes low voltage unable to spark the cylinder to ignite the vehicle. A blue spark is a strong spark with enough energy to ignite the car.
Final Words
Ignition coils are essential electromagnetic devices that convert the 12V from a car battery to around 100000V. 100000V is enough to power the sparking of the cylinders in the engine to ignite and facilitate fuel combustion for the car to operate.
Any problems with the ignition coils can invoke severe issues with vehicle operations. That is why car owners need to monitor any symptoms of a bad ignition coil. These symptoms include engine misfires, hard starts, poor mileage control, and backfires.
Luckily, the replacement procedure for ignition coils is simple, as outlined earlier. Don’t wait for your car to develop worse problems. Fix it as soon as you spot the symptoms outlined.