Your car’s ignition coil might not be the flashiest part of your engine, but it plays a crucial role in keeping your ride running smoothly. Think of it as the unsung hero that transforms your car battery’s voltage into the spark your engine needs to fire up and run. Without it, you’re not going anywhere fast.
So, what happens when an ignition coil starts to fail? You might notice your engine stuttering, check engine light glowing on your dash, or even poor gas mileage making a dent in your wallet. Ignoring the signs of a bad ignition coil can lead to bigger problems, like damaging your catalytic converter or leaving you stranded.
In this article, we’ll break down what an ignition coil does, the most common signs of failure, and how to address the issue.
Table of Contents
Common Signs of a Bad Ignition Coil
What Causes Ignition Coil Failure?
How to Diagnose a Bad Ignition Coil
What is an Ignition Coil?
An ignition coil might sound fancy, but it’s just a hardworking little transformer that’s vital for your car’s ignition system. Its main job? Taking the low voltage from your car’s battery and cranking it up—way up—so your spark plugs can ignite the air-fuel mixture in your engine. No spark, no boom, no go.
Here’s how it works: Your battery supplies 12.65 volts, which is great for powering your radio or headlights but nowhere near enough to jumpstart the combustion process. Enter the ignition coil. It boosts that voltage up to a jaw-dropping 20,000 to 60,000 volts (in some cars, a lot more), giving your spark plugs the zap they need to fire at just the right time.
Ignition coils come in a few different styles. Older cars typically have a single coil that serves the entire engine, while newer rides sport coil packs or a coil-on-plug setup—one coil per cylinder. The advantage? More precise control, better efficiency, and fewer chances for things to go wrong.
But like any hardworking component, ignition coils wear out. Heat, vibration, and electrical stress can take their toll, leaving your engine struggling to keep up. That’s why spotting the signs of a failing ignition coil early is key to avoiding bigger spendier problems.
Now that you know what an ignition coil does, let’s dive into the symptoms that could mean yours is on its last legs.
Common Signs of a Bad Ignition Coil
When an ignition coil starts to fail, your car won’t stay quiet about it. From misfires to weird smells, your vehicle throws out plenty of red flags—you just need to know what to look for. Here are the most common symptoms that your ignition coil might be on its way out:
1. Engine Misfires
Car may feel jerky. Misfires happen when one or more cylinders fail to ignite properly, and a weak or failing ignition coil is a prime suspect. Left unchecked, those misfires can damage your catalytic converter—a problem you don’t want; cats are expensive.
2. Check Engine Light
A faulty ignition coil often triggers the check engine light, and if you scan it with an OBD-II tool, you’ll probably see misfire-related codes (P0300-P0308). This is your car’s way of telling you, “Something’s up—check it out!”
3. Poor Fuel Economy
Spending more time at the gas pump? A bad ignition coil can cause incomplete combustion, forcing your engine to work harder and burn more fuel.
4. Difficulty Starting
When your ignition coil isn’t delivering a strong enough spark, starting your engine can feel like a chore. You might hear the engine crank but fail to catch, especially in cold weather or after sitting idle for a while.
5. Loss of Power and Acceleration
If your car feels sluggish and unresponsive when you hit the gas, it could be your ignition coil struggling to keep up. Weak sparks mean weaker engine performance, and you’ll feel it every time you try to accelerate.
6. Stalling
Your car suddenly shuts off while idling or cruising? That’s a classic sign of an intermittent ignition coil failure. It’s not just inconvenient—it can be downright dangerous, especially in traffic.
7. Backfiring or Fuel Smell
If you hear a loud pop from your exhaust or notice a strong smell of unburned fuel, your ignition coil could be the culprit. When it fails, unburned fuel can leak into the exhaust system, causing backfires or that distinct gasoline aroma.
8. Rough Idling
Your car might sound like it’s purring when it’s running smoothly, but when the ignition coil is bad, the engine might sputter or shake while idling.
Up next, we’ll look at what causes ignition coils to fail and how you can diagnose the problem.
What Causes Ignition Coil Failure?
Ignition coils are built to handle a lot, but they’re not invincible. Over time, wear and tear—or a little bad luck—can push them past their breaking point. Here’s a look at the usual suspects when it comes to ignition coil failure:
1. Heat and Vibration
Your ignition coil lives in a pretty harsh environment. Between the constant heat from the engine and the vibrations from driving, it’s under a lot of stress. Over time, this can cause cracks in the coil’s housing, which lets moisture and contaminants sneak in, leading to failure.
2. Moisture or Oil Contamination
Ever popped the hood and noticed oil pooling around the spark plugs? A leaky valve cover gasket can let oil seep into places it shouldn’t be, including the ignition coil. Moisture is another killer—water sneaking in during a wash or heavy rain can wreak havoc on the coil’s internal wiring.
3. Faulty Spark Plugs
Here’s the thing: ignition coils and spark plugs are a team. If the spark plugs are worn out or gapped incorrectly (gap too big), the coil has to work overtime to produce a stronger spark. That extra load can cause the coil to overheat and fail prematurely.
4. Old Age
Nothing lasts forever—not even your ignition coil. If your car has been around the block a few times (or a few hundred thousand miles), the coil might simply be at the end of its lifespan. Age-related wear is one of the most common causes of failure.
5. Poor Quality Parts
Opting for cheap aftermarket parts might save you a few bucks upfront, but they often don’t last as long. Low-quality ignition coils are more prone to failure, so sticking with OEM or high-quality replacements is always a smart move.
6. Carbon Tracking
Carbon tracking happens when electrical energy follows a path across the spark plug insulator instead of jumping the gap. This creates a weak or misdirected spark, forcing the ignition coil to work harder than it should. Over time, this extra strain can cause it to fail.
7. Engine Misfires and Ignition Timing Issues
If your engine is already misfiring (not coil-related) or your ignition timing is off, it creates a feedback loop that stresses the ignition coil even more.
In the next section, we’ll cover how to replace a bad ignition coil and get your engine back on track.
How to Diagnose a Bad Ignition Coil
Figuring out if your ignition coil is on the fritz doesn’t have to be rocket science. With a little know-how and some basic tools, you can narrow down the issue without tearing your whole engine apart. Here’s how to get started:
1. Look for Trouble Codes
The check engine light isn’t just there to ruin your day—it’s your car’s way of saying, “Hey, something’s wrong.” Grab an OBD-II scanner and plug it into your car’s diagnostic port. A bad ignition coil will often trigger codes like P0300 (random misfire) or P0301-P0308 (misfires in specific cylinders). These codes can give you a solid clue about which cylinder—and possibly which coil—is acting up.
2. Do a Visual Inspection
Pop the hood and take a good look at the ignition coils. Signs of trouble include cracks in the housing, burn marks, or corrosion on the electrical connectors. If you see any of these, you might have found your culprit.
3. Swap the Coils
This is an old-school but highly effective diagnostic trick. If your OBD-II scanner flagged a misfire in, say, cylinder 2 (P0302), start by swapping the ignition coil from that cylinder with one from another cylinder. Clear the trouble codes, fire up the engine, and monitor for new codes. If the misfire shifts to the cylinder you moved the coil to, bingo—you’ve confirmed the coil is faulty and needs replacing.
If the misfire stays with cylinder 2, the ignition coil isn’t the culprit, so it’s time to dig deeper. The next step is to pull the spark plug and check its condition. Look for signs of fouling, wear, or improper gapping. If the plug looks suspicious, swap it with one from another cylinder, clear the codes, and test again. If the misfire follows the spark plug to the new cylinder, you’ve found your problem—a faulty spark plug.
If the misfire still sticks with cylinder 2, you’re likely dealing with something beyond the ignition system. Possible culprits include:
- A clogged or failing fuel injector: Preventing proper fuel delivery to the cylinder.
- Carbon buildup: Causing poor combustion.
- Valve issues: Such as a burned or leaking valve, disrupting airflow.
- Head gasket problems: Allowing coolant or oil to seep into the combustion chamber.
- Compression issue: Wear and tear on the cylinder, piston and rings.
At this point, further testing is required. A compression test, leak-down test, or injector test can help pinpoint the issue. While electrical problems are often the first suspect, mechanical issues can cause similar symptoms.
4. Check for Spark
If your car has a coil-on-plug setup, you can use a spark tester to confirm whether the coil is functioning properly. Here’s how to do it step-by-step:
- Remove the Coil
Carefully disconnect the ignition coil from the spark plug and wiring harness. - Attach the Adjustable Spark Tester
Connect the spark tester to the coil, ensuring it’s securely attached and connect the coil wiring harness plug connector. - Set the Spark Tester Gap
- For a standard misfire diagnosis, set the gap to approximately 1 cm (30kV).
- For an intermittent misfire or to test coil strength under stress, set it to 2 cm (60kV).
- Ground the Spark Tester
Attach the tester’s ground clip to a bare metal surface on the engine. This provides a solid ground connection. - Disable Fuel Delivery to the Cylinder
- Remove the fuel pump relay or disconnect the injector wiring plug for the tested cylinder. This prevents fuel from entering the combustion chamber.
- Note: Disconnecting the injector will trigger a fuel-related fault code. You’ll need to clear this code later with an OBD-II scanner.
- Crank the Engine
Have a helper crank the engine while you observe the spark tester. Look through the tester’s window for a consistent, strong spark. - Interpret the Results
- No Spark: The coil is likely faulty, though you should also inspect the wiring and connections for issues.
- Weak or Intermittent Spark: The coil may still be the problem, but check for other possible issues like a bad spark plug or a weak electrical supply.
This quick test is a reliable way to confirm if an ignition coil is doing its job—or if it’s time to replace it.
5. Check coil wiring
Sometimes the coil isn’t the problem—it’s the wiring. Damaged, loose, or corroded connectors can interrupt the signal from the ECU to the coil. Give the wiring harness a good once-over to make sure everything is snug and intact.
To diagnose a typical 3-wire ignition coil setup, follow these steps to test the power, ground, and signal wiring. You can use a test light or a voltmeter:
- Prepare for Testing
- Remove the fuel pump relay to prevent the engine from starting during the test.
- Disconnect the coil’s plug terminal to access the wiring.
- Check for Power
- Turn the ignition to the “On” position (engine off).
- Use the test light or voltmeter to probe the terminals in the connector.
- One of the terminals should have power.
- No Power: This points to a possible wiring issue or a blown fuse.
- Check for Ground
- With the ignition still in the “On” position, test the terminals for a ground connection.
- If using a test light, connect the test light’s ground clip to the positive side of the battery. When the test light illuminates, you’ve found a good ground.
- No Ground: This indicates a possible wiring issue or a faulty ground connection.
- Check for Signal
- Have a helper crank the engine while you probe the terminals.
- One of the terminals should show an intermittent signal (pulsing voltage).
- No Signal: This could mean a wiring problem or a faulty ECM driver.
In a four-wire coil wiring harness, the fourth wire typically sends a signal from the coil back to the ECM to indicate it has fired, so you won’t get a signal on this wire unless testing with the coil fitted and firing.
By methodically testing each connection, you can identify whether the problem lies in the wiring, the ECM, or the coil itself.
6. Rule Out Other Issues
Here’s the thing: symptoms of a bad ignition coil can overlap with other problems, like failing spark plugs or a clogged fuel injector. Before you commit to replacing a coil, double-check these potential culprits to avoid throwing money at the wrong fix.
By working through these steps methodically, you can pinpoint whether a bad ignition coil is the source of your engine troubles. And once you know for sure, fixing it is usually a straightforward job—so you can get back to smooth driving without breaking a sweat.
Ignition Coil FAQs
Got questions about ignition coils? You’re not alone. These little powerhouses are essential to keeping your engine running smoothly, but they can also cause a lot of confusion when things go wrong. Here are the most common questions people ask—and the answers to help you sort it out.
Can I drive with a bad ignition coil?
Technically, yes, but it’s not a great idea. Driving with a failing ignition coil can lead to misfires, which can damage your catalytic converter—a costly repair. Plus, it’ll kill your fuel economy and might even leave you stranded.
How long do ignition coils last
On average, ignition coils last around 100,000 miles, but this can vary depending on the quality of the part and your driving habits. Regular maintenance, like replacing spark plugs on time, can help them last longer.
How much does it cost to replace an ignition coil?
The cost depends on your car and how many coils need replacing. For a single coil, you’re looking at anywhere from $50 to $150 for the part and around $100 to $200 for labor if you’re not doing it yourself. Coil pack setups can cost more since there are multiple coils.
Do I need to replace all ignition coils if one fails?
Not always, but it’s often a good idea. If one coil has gone bad, the others might not be far behind—especially if they’re the same age. Replacing them all at once can save you from future breakdowns and labor costs.
Can a bad ignition coil damage my engine?
Indirectly, yes. A bad coil can cause misfires, which lead to unburned fuel entering the catalytic converter. This can cause the converter to overheat and fail, and that’s a repair you definitely want to avoid.
Can I replace an ignition coil myself?
Absolutely! Replacing an ignition coil is usually a straightforward DIY job, especially if you’ve got a coil-on-plug setup. Just make sure you have the right tools, follow the steps in your car’s repair manual, and don’t forget to double-check the firing order when reassembling.
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- About the Author
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.