What an ignition coil does and why it fails
An ignition coil takes the low voltage from your battery and converts it into the high voltage needed to fire your spark plugs. When it fails, your engine won't start, misfires under load, or runs rough at idle. A bad coil is one of the most common causes of a check engine light.
Ignition coils fail because they get hot every time the engine runs, and that heat eventually breaks down the insulation inside. Moisture, oil leaks, and vibration speed up the process. Most coils last 80,000 to 150,000 miles, but some fail much earlier if they're exposed to water or if your spark plugs are worn out and drawing too much voltage.
You can test a coil yourself with a multimeter or by swapping it with a known good one. Neither test is difficult, and both will tell you whether the coil is the problem or whether you should look elsewhere.
Key Takeaways
- A failing ignition coil causes hard starts, rough idle, misfires under acceleration, or a check engine light with a misfire code.
- You can test a coil with a multimeter by checking its primary and secondary resistance, or by swapping it with a coil from another cylinder to see if the misfire moves.
- Resistance readings vary by vehicle make and model, so you need the correct spec from your service manual or a repair database before you test.
- If the coil tests bad or a swap test moves the misfire to a different cylinder, the coil is the problem and needs replacement.
- Ignition coils are usually inexpensive and straightforward to replace, but the labor cost depends on how many coils your engine has and where they sit.
Gather the information you need before testing
Before you pull out a multimeter, find the correct resistance specifications for your vehicle. Ignition coil resistance varies widely—a coil that reads 0.8 ohms on one engine might be dead at 0.8 ohms on another. You need the exact spec for your make, model, and year.
Check your vehicle's service manual first. If you don't have one, search online for "[your year, make, model] ignition coil resistance spec" or look it up in a repair database like AllData or Mitchell1 (many public libraries offer free access). Write down both the primary resistance (the low-voltage side) and secondary resistance (the high-voltage side). Primary resistance is usually between 0.4 and 2 ohms; secondary is usually between 5,000 and 15,000 ohms, but this varies widely.
Also note which cylinder each coil fires. On older engines, one coil fires two cylinders. On newer engines, each spark plug usually has its own coil sitting on top of it. Knowing the layout helps you identify which coil to test first and whether you can swap coils to move the misfire.
Test the coil with a multimeter
Set your multimeter to the ohms (resistance) setting. Disconnect the ignition coil's electrical connector—do not test while the engine is running. If the coil is bolted to the spark plug, you may need to remove it first; if it's mounted on the valve cover or engine block, you can test it in place.
For the primary resistance test, touch the multimeter probes to the two primary terminals on the coil (the terminals where the battery wires connect). The reading should fall within the range you found in your service manual. If it reads zero or infinity (no continuity), the coil is open internally and has failed. If it reads far outside the spec range, it has likely failed.
For the secondary resistance test, touch one probe to a primary terminal and the other to the high-voltage output terminal (where the spark plug wire or boot connects). Again, compare your reading to the spec. A reading of zero, infinity, or far outside the range means the coil has failed. If both primary and secondary readings are within spec, the coil is likely good and the problem lies elsewhere—bad spark plugs, a fuel injector, a compression issue, or a sensor fault.
Write down your readings. If you test multiple coils, note which one is out of spec. That coil needs replacement.
Use the swap test if you don't have a multimeter
If you don't have a multimeter or prefer a simpler test, swap the suspected coil with a coil from another cylinder. This works best on engines where each spark plug has its own coil, or where you can easily move coils between cylinders.
First, note which cylinder is misfiring. Your check engine light will show a code like P0300 (random misfire), P0301 (cylinder 1 misfire), or P0302 (cylinder 2 misfire). Write down the cylinder number. Then, remove the coil from that cylinder and swap it with the coil from a different cylinder—one that is running smoothly. Reinstall both coils, clear the check engine light with a scanner, and drive the vehicle.
If the misfire code now appears for the cylinder where you moved the bad coil, the coil you swapped out is bad. If the misfire code still shows the original cylinder, the problem is not the coil—it's the spark plug, fuel injector, compression, or a sensor in that cylinder. If the misfire disappears entirely, something else was wrong, and swapping the coils fixed it temporarily (this is rare and usually means a loose connection).
What to do if the coil is bad
Once you've confirmed the coil is bad, you have two choices: replace it yourself or take it to a shop. Ignition coils are usually between $40 and $150 each, depending on the vehicle. Replacement is straightforward on most cars—you disconnect the electrical connector, unbolt the coil, and bolt on the new one. The whole job takes 10 to 30 minutes per coil.
If you're not comfortable working on your engine, a shop will charge labor on top of the coil cost. Labor typically ranges from $75 to $200 per coil, depending on how many coils your engine has and how accessible they are. Some engines have coils buried under intake manifolds or other components, which drives the labor cost up.
When you replace a coil, consider replacing the spark plugs at the same time if they're due. A worn spark plug draws extra voltage and can burn out a new coil prematurely. If you're replacing multiple coils, replace all of them together—if one has failed from age or heat, the others are likely close behind.
Common reasons the test gives confusing results
If your resistance readings are borderline—close to the spec but not quite in range—the coil may be failing but not yet completely dead. A coil that reads 0.9 ohms when the spec says 0.5 to 0.8 ohms is degrading and will likely fail soon. You can drive on it for now, but plan to replace it within the next few weeks.
If the multimeter reads infinity (no continuity) on the secondary test but the primary reads normal, the coil's secondary winding is open. This is a failure. If both readings are normal but you still have a misfire, the coil is not the problem—check the spark plug, fuel injector, or compression in that cylinder.
Temperature affects resistance slightly. A coil that's been running will read differently than a cold coil. If you get a borderline reading, let the engine cool for 30 minutes and test again. If the reading improves significantly, the coil is overheating and needs replacement. If it stays the same, the coil is genuinely failing.
Frequently Asked Questions
Can a bad ignition coil damage other parts of my engine?
A failing coil won't damage the engine itself, but it can damage the catalytic converter if the misfire goes unchecked for thousands of miles. Unburned fuel enters the exhaust and heats the converter to dangerous temperatures. Replace the coil as soon as you confirm it's bad to avoid a much more expensive repair.
Should I replace all the ignition coils at once?
If only one coil is bad, you only need to replace that one. However, if your vehicle has high mileage and multiple coils are showing resistance readings near the edge of spec, replacing them all together saves labor and prevents a second failure a few months later. Ask your mechanic whether the other coils look worn.
What if I test the coil and it reads normal, but I still have a misfire?
The problem is not the coil. Check the spark plug for wear or fouling, test the fuel injector for proper spray pattern, or have a compression test done on that cylinder. A bad sensor—oxygen sensor, mass airflow sensor, or cam timing sensor—can also cause a misfire code without the coil being at fault.
Can I test an ignition coil while the engine is running?
No. Disconnect the coil and let the engine cool before testing. Testing a live coil risks damage to your multimeter and can cause injury from the high voltage the coil produces.
How do I know if my coil is failing before the check engine light comes on?
Listen for a rough idle, hesitation under acceleration, or a slight loss of power. These are early signs of a coil beginning to fail. If you notice them, have the coil tested before it fails completely and leaves you stranded.