What an ignition coil does and when it fails
Your ignition coil converts the battery's low voltage into the high voltage your spark plugs need to fire. When it fails, your engine won't start, or it will start and then stall or misfire while you drive. You might also see a check engine light on your dashboard.
A failing coil usually shows up as rough idling, hesitation when you accelerate, or a noticeable loss of power. Some cars have one coil for all cylinders; others have one coil per cylinder. If you have one coil per cylinder and only one fails, you might notice the problem only under load — like when you're climbing a hill or merging on the highway.
Ignition coils typically last 80,000 to 150,000 miles, though some fail earlier if they overheat or get wet. A mechanic can confirm a bad coil by reading your engine codes with a diagnostic scanner, or by testing the coil's resistance with a multimeter.
Key Takeaways
- Most ignition coils bolt directly to the spark plug or sit on top of the engine, making them one of the simpler engine components to replace yourself.
- You will need a socket set, a wrench, and possibly a coil puller tool, depending on how tightly your coil is seated.
- Replacing a coil takes 15 minutes to an hour per coil, depending on how many you have and how accessible they are in your engine bay.
- New coils cost between $40 and $150 each at an auto parts store, and labor at a shop typically runs $100 to $300 per coil including diagnosis.
Locate your ignition coil and get the right replacement
Open your hood and look for a cylindrical or rectangular component sitting directly on top of a spark plug, or mounted on the valve cover or engine block nearby. On older cars, there may be only one large coil with thick wires running to each spark plug. On newer cars, you'll see smaller coils mounted right on top of each spark plug — these are called coil-on-plug (COP) coils.
Write down your vehicle's year, make, model, and engine size. Take a photo of the coil with your phone. Go to an auto parts store or search online using that information, and order the exact coil for your car. Do not guess at the part number — coils are not interchangeable across different engines, and installing the wrong one will not work.
If you're replacing a coil-on-plug, you only need to replace the one that failed. If you have an older single coil with plug wires, you're replacing the whole unit. Some shops recommend replacing all coils at once if one has failed, since they age together, but you can replace them one at a time if cost is a concern.
Disconnect the battery and remove the old coil
Disconnect the negative terminal of your battery by loosening the nut with a wrench and sliding the cable off. This prevents electrical shock and protects your car's computer while you work.
Locate the electrical connector on the coil — it's a plastic plug with wires running into it. Press the tab or clip on the connector and pull it straight out. If it's stuck, wiggle it gently side to side rather than yanking straight back.
Unbolt the coil using a socket wrench. Most coils have one or two bolts holding them in place. If the coil is stuck tight on the spark plug, a coil puller tool (available at auto parts stores for $10 to $20) will save you time and frustration. Without one, you can tap the coil gently with a rubber mallet to break the seal, then twist and pull it straight up and off the spark plug.
Install the new coil and reconnect everything
Before you install the new coil, look at the rubber boot on the end that sits on the spark plug. If it's cracked or hardened, replace it — it costs a few dollars and prevents moisture from getting into the spark plug well. Slide the new rubber boot onto the spark plug if you're using a separate one.
Lower the new coil straight down onto the spark plug, making sure it seats fully. The coil should sit flush against the engine. Install the bolts and tighten them snugly — do not over-tighten, as you can crack the coil housing. Most coils need only 15 to 25 foot-pounds of torque, which feels like hand-tight plus a quarter turn with a wrench.
Plug the electrical connector back in until you hear or feel it click. Reconnect the negative battery terminal by sliding the cable back onto the post and tightening the nut.
Start the engine and clear any warning lights
Turn the key to start the engine. It should start normally and idle smoothly. If it starts but runs rough, or if the check engine light stays on, turn it off and double-check that the connector is fully seated and the coil bolts are tight.
If the light stays on after a few restarts, the problem may not have been the coil, or there may be a secondary issue like a bad spark plug or fuel injector. A mechanic with a diagnostic scanner can read the code and tell you what's happening.
The check engine light may take a few driving cycles to turn off on its own. If it doesn't, some auto parts stores will clear it for free using a code reader, or you can disconnect the battery for 15 minutes to reset the computer — though this will also erase any other stored codes a mechanic might need to see.
When to replace spark plugs at the same time
While you have the coil off, look at the spark plug underneath. If it's black and sooty, or if the gap (the space between the electrodes) looks too wide, replace the plug now. You're already halfway there, and a new plug costs $3 to $10.
If your spark plugs are due for replacement anyway — most cars need new plugs every 30,000 to 100,000 miles depending on the type — doing it while the coil is off saves you time. Unscrew the old plug with a spark plug socket, check the gap on the new plug against your owner's manual, and screw the new one in by hand first to avoid cross-threading.
Frequently Asked Questions
Can I drive with a bad ignition coil?
You can drive short distances, but the engine will misfire and you'll lose power. Continuing to drive on a bad coil can damage your catalytic converter, which is expensive to replace. If your check engine light is on and you suspect a coil, have it diagnosed before driving long distances.
How do I know if it's the coil and not something else?
A mechanic with a code reader can tell you exactly which cylinder is misfiring. If the code points to a specific coil, that's your answer. You can also test the coil's resistance with a multimeter — a dead coil will show no continuity or very high resistance compared to the manufacturer's spec in your service manual.
Do I need special tools to replace a coil?
A socket set, wrench, and screwdriver are usually enough. A coil puller tool makes removal easier if the coil is stuck, but you can work around it with a rubber mallet and patience. Some coils sit in tight spaces and may need a small ratchet wrench to reach the bolts.
What if I replace the coil and the light comes back on?
The misfire code may point to a different problem — a worn spark plug, a fuel injector that's not spraying properly, or a compression issue in that cylinder. Have the code read again to see if it's the same cylinder or a different one. If it's the same cylinder, the coil may not have been the root cause.
Can I replace just one coil or do I have to replace them all?
You can replace just the failed coil. However, if all your coils are the same age and mileage, they may all be near the end of their life. Some owners replace them all at once to avoid repeat trips under the hood, but it's not required.