What an ignition coil does and why it fails

An ignition coil converts the low voltage from your car's battery into the high voltage needed to fire the spark plugs. Without it, your engine cannot ignite the fuel in the cylinders, so your car will not start or will run roughly and stall. Most cars have one coil per spark plug, though some older vehicles have a single coil serving all plugs.

Ignition coils fail because of heat, vibration, and electrical stress. They typically last 80,000 to 150,000 miles, though this varies by vehicle and driving conditions. A failing coil usually shows up as a check engine light, rough idling, misfiring (the engine stumbling under acceleration), or difficulty starting. Sometimes only one coil fails while others still work, which is why you might notice the problem only under certain conditions.

You can replace a coil yourself if you are comfortable working under the hood and have basic hand tools. The job usually takes 15 to 30 minutes per coil. If you are not confident, a mechanic can do it in under an hour, though labor costs vary widely by shop and location.

Key Takeaways

  • An ignition coil converts battery voltage to spark plug voltage, and a bad one causes rough running, misfiring, or a check engine light.
  • You will need the correct replacement coil for your specific vehicle year, make, and model — buying the wrong one wastes money and time.
  • The replacement process involves disconnecting the battery, removing the old coil, and installing the new one in the same location, with no special tools required in most cases.
  • After installation, the check engine light may stay on until you clear the code with a scanner or drive the car for a few cycles so the computer resets.

Finding the right replacement coil for your vehicle

The first step is knowing which coil you need. Ignition coils are not universal — a coil for a 2015 Honda Civic will not work in a 2015 Ford Focus. You need the year, make, model, and engine size of your vehicle. This information is on your registration, in your owner's manual, or on a sticker inside the driver's door jamb.

Once you have that information, search for the part number online using your vehicle details. Auto parts retailers like AutoZone, O'Reilly, NAPA, and Advance Auto Parts all have search tools on their websites. You can also call a dealership parts department and give them your vehicle information — they will tell you the exact part number and price. Aftermarket coils usually cost $25 to $75 each, while OEM (original equipment manufacturer) coils from the dealership cost $50 to $150 or more.

Buy the coil before you start work so you have it on hand. Some retailers offer same-day pickup at a local store, which is faster than waiting for shipping. If you are unsure whether the coil is the problem, many auto parts stores will read your check engine code for free to confirm a coil misfire before you buy anything.

Locating the ignition coils on your engine

Ignition coils sit on top of or very near the spark plugs. Open your hood and look for a row of cylindrical or rectangular black plastic components connected to thick wires or sitting directly on the spark plug wells. Some vehicles have coils mounted on the side of the engine block or on the firewall instead.

Your owner's manual or a service diagram will show you exactly where the coils are located on your specific engine. You can also search online for "[your vehicle year, make, model] ignition coil location" to find photos or videos showing the exact spot. If you have multiple coils and are not sure which one is bad, the check engine code will tell you — it will say something like "P0301" (cylinder 1 misfire) or "P0302" (cylinder 2 misfire), which tells you which coil to replace.

Mark the location of the coil you are removing before you disconnect anything. If you are replacing multiple coils, take a photo of the engine bay so you remember which wire or connector goes where when you reinstall them.

Removing the old coil step by step

Start by disconnecting the negative terminal of your battery using a wrench. This prevents electrical shock and protects the engine computer. Wait a few minutes after disconnecting the battery so any residual charge drains away.

Next, disconnect the electrical connector from the coil. This is usually a clip or latch that you squeeze or pull to release. Gently wiggle the connector side to side as you pull — do not yank it straight out, as this can damage the pins inside. If the connector is stuck, spray a small amount of penetrating oil around the base and wait a minute before trying again.

If your coil has a thick wire running to the spark plug (an older design), carefully twist the wire boot at the base where it connects to the coil, then pull it straight out. Do not pull the wire itself, as this can damage the internal conductor.

Now remove the bolt or bolts holding the coil in place. Most coils have one or two bolts that screw into the engine block or cylinder head. Use a socket wrench or box wrench to remove them — the size is usually 10mm or 13mm. Keep the bolts in a safe place so you do not lose them. Lift the coil straight up and out of its mounting location.

Installing the new coil and reconnecting everything

Before you install the new coil, inspect the mounting location for dirt, oil, or corrosion. Wipe it clean with a dry cloth. If the mounting surface is corroded or pitted, you can lightly sand it with fine-grit sandpaper to improve the connection, though this is not always necessary.

Position the new coil in the same location as the old one and insert the bolts. Tighten them firmly but do not over-tighten — you want them snug enough that the coil does not move, but over-tightening can crack the coil housing or strip the threads. A typical torque specification is 15 to 25 foot-pounds, though your manual will have the exact number for your vehicle.

Reconnect the electrical connector by pushing it onto the coil until you hear or feel a click. If there was a wire boot, push it back onto the spark plug terminal and twist it clockwise until it seats. Reconnect the negative battery terminal by tightening the wrench on the bolt.

Start the engine and listen for any unusual sounds. The check engine light may still be on — this is normal. You can clear the code yourself with an OBD-II scanner (a device that plugs into the diagnostic port under your steering wheel), or you can drive the car normally for a few days and the light will usually turn off on its own once the computer confirms the problem is fixed.

When to replace multiple coils at once

If you have misfires on more than one cylinder, you may need to replace multiple coils. However, before you buy several coils, confirm that the other cylinders actually have bad coils and not a different problem like a bad spark plug, fuel injector, or compression issue. A mechanic or a detailed diagnostic scan can tell you which cylinders are misfiring.

If coils are failing on multiple cylinders, it is often worth replacing all of them at the same time, even if only one or two are showing codes. Coils that are about to fail usually fail around the same time because they experience the same heat and electrical stress. Replacing them all at once saves you from having to do the job again in a few months. The total cost of replacing four coils is usually $100 to $300 in parts, depending on your vehicle.

Some mechanics recommend replacing spark plugs at the same time you replace coils, since you already have straightforward access to the spark plug wells. This is optional but can save labor costs if you were planning to replace them soon anyway.

Troubleshooting if the problem persists after replacement

If you replace the coil and the check engine light comes back on, or the engine still misfires, the problem may not have been the coil. Other common causes of misfire codes are a bad spark plug, a clogged fuel injector, low fuel pressure, a vacuum leak, or a compression problem in that cylinder.

If the new coil is the wrong part number, it will not fit properly or will not work at all. Double-check that you ordered the correct coil for your vehicle year, make, and model. If you bought it from an auto parts store, they will exchange it for the correct one if you bring your vehicle information and receipt.

If the coil is correct and installed properly but the problem continues, take the car to a mechanic for a full diagnostic. They can test fuel pressure, compression, and the fuel injectors to find the real cause. Continuing to drive with a misfire can damage the catalytic converter, which is an expensive repair, so it is worth getting it checked if the new coil does not solve the problem.

Frequently Asked Questions

Can I drive the car with a bad ignition coil?

You can drive it, but you should not for long. A bad coil causes misfiring, which wastes fuel and can overheat the catalytic converter. Continuing to drive can cause permanent damage to the converter, which costs $400 to $1,200 to replace. Replace the coil as soon as you can.

How do I know if I have a bad coil versus a bad spark plug?

A check engine code will tell you which cylinder is misfiring. If the code points to one specific cylinder, it is usually the coil or spark plug for that cylinder. You can swap the coil from a cylinder that is running fine into the bad cylinder — if the misfire moves to the other cylinder, the coil is bad. If the misfire stays, the spark plug is likely the problem.

Do I need to clear the check engine code after I replace the coil?

No, but you can if you want to. The light will turn off on its own after the engine computer runs a few diagnostic cycles and confirms the problem is gone, usually within a few days of normal driving. If you want it off when ready, you can use an OBD-II scanner to clear the code yourself, or a mechanic can do it for you.

What is the difference between OEM and aftermarket ignition coils?

OEM coils are made by the vehicle manufacturer and usually cost more but come with a longer warranty. Aftermarket coils are made by third-party companies and cost less but may not last as long. Both will work, and most people choose aftermarket to save money unless they have had problems with aftermarket parts in the past.

Can a bad ignition coil damage other parts of the engine?

Yes. A coil that misfires sends unburned fuel into the exhaust system, which can overheat and damage the catalytic converter. This is why it is important to replace a bad coil quickly rather than ignoring the check engine light.