What an ignition coil does and why it fails
An ignition coil converts your battery's low voltage into the thousands of volts needed to fire your spark plugs. When it fails, your engine won't start, or it will start and then misfire — running rough, losing power, or stalling without warning. A bad coil can also trigger a check engine light.
Ignition coils fail because of heat, moisture, and electrical stress. They sit near the engine where temperatures climb, and they handle high voltage every time you start the car. Most coils last 80,000 to 150,000 miles, but some fail much earlier if the engine runs hot or if spark plugs are worn out and forcing the coil to work harder.
Before you test, confirm the coil is actually the problem. A failing coil usually shows one or more of these signs: the engine won't turn over at all, it starts but misfires under load, you see a misfire code (P0300 through P0308) on a scan tool, or the coil itself is visibly cracked or wet inside.
Key Takeaways
- A visual inspection — looking for cracks, corrosion, or moisture inside the coil — catches many failures without tools.
- A multimeter set to resistance mode can test whether the coil's internal windings are open or shorted, which takes five minutes per coil.
- A spark tester (a cheap tool that clips to the spark plug wire) shows whether the coil is actually producing spark when the engine cranks.
- Modern cars with multiple coils (one per cylinder) let you swap a suspected bad coil to a different cylinder to see if the misfire moves with it.
- If the coil tests bad, replace it; if it tests good but you still have a misfire, the problem is likely the spark plug, fuel injector, or compression in that cylinder.
Visual inspection: what to look for before testing
Start by looking at the coil itself. On older cars, the coil is a single large canister mounted on or near the engine block. On newer cars, there is usually one small coil sitting directly on top of each spark plug (called a coil-on-plug design). Either way, remove any wires or connectors gently and examine the coil's exterior and interior.
Look for cracks in the plastic housing, white or green corrosion on the terminals, or moisture pooled inside. Moisture is a common killer — if water has gotten into the coil, it will fail. A cracked coil cannot be repaired and must be replaced. If you see obvious damage, you have found your problem and do not need to test further.
Also check the spark plug wire or connector. On older coils, the wire should be dry and the rubber boot (the connector end) should not be cracked or loose. On coil-on-plug designs, the connector should click firmly into place and show no corrosion inside the terminal. A loose or corroded connection can mimic a bad coil.
Testing with a multimeter: checking resistance
A multimeter measures the electrical resistance inside the coil's windings. A coil with broken or shorted windings will show an out-of-range reading. You will need a multimeter set to the ohms (resistance) setting — most auto parts stores rent them for free or sell a basic one for $15 to $25.
Disconnect the coil from the car by unplugging the electrical connector. On coil-on-plug designs, you may need to remove a bolt or clip to lift the coil off the spark plug. Once it is free, touch one multimeter probe to one terminal of the coil and the other probe to the second terminal. Write down the reading.
Compare your reading to the coil's specification. This number varies by make and model — a typical range is 0.4 to 2 ohms for primary resistance and 5,000 to 15,000 ohms for secondary resistance. Your owner's manual or a service manual for your car will list the exact range. If your reading falls outside that range, the coil is bad. If it is within range, the coil's windings are intact and you need to test further or look elsewhere for the problem.
Testing with a spark tester: checking for actual spark
A spark tester is a small tool (usually $10 to $20) that clips onto a spark plug wire or coil-on-plug connector and lights up when high voltage passes through it. It shows whether the coil is actually firing, not just whether its resistance is correct. Some coils can pass a resistance test but still fail to produce spark under load.
Attach the spark tester to the spark plug wire or coil connector. Have someone crank the engine while you watch the tester. If it flashes brightly with each crank, the coil is producing spark. If it does not flash at all, or flashes very dimly, the coil is likely bad. If it flashes on some cranks but not others, the coil may be failing intermittently — a sign it is about to fail completely.
A spark tester is more reliable than a multimeter for catching coils that fail under the stress of actually firing. If the tester shows no spark but the multimeter showed good resistance, the coil is still bad — it just failed in a way the resistance test did not catch.
Swapping coils to isolate the problem
If your car has multiple coils (one per cylinder, common on cars made after 2000), you can swap a suspected bad coil to a different cylinder and see if the misfire follows it. This test works because if the coil is bad, the misfire will move to whichever cylinder the bad coil is now on.
First, note which cylinder the misfire is happening on. Your scan tool will show this as a code like P0301 (cylinder 1 misfire) or P0304 (cylinder 4 misfire). Unplug the coil from that cylinder and move it to a different cylinder — usually cylinder 2 or 3. Plug it in firmly, start the engine, and scan for codes again.
If the misfire code now points to the cylinder where you moved the coil, the coil is bad. If the code still points to the original cylinder, the problem is not the coil — it is the spark plug, fuel injector, or engine compression in that cylinder. This test takes ten minutes and saves you from replacing a good coil.
What to do if the coil tests bad
If your testing shows the coil is bad, you will need to replace it. Coils are not repaired — they are swapped out. The cost varies widely: a replacement coil runs $40 to $150 depending on the make and model, and labor at a shop adds another $50 to $200. If you are comfortable removing bolts and connectors, you can replace it yourself in 15 to 30 minutes.
When you buy a replacement, match the part number to your car's year, make, model, and engine size. Do not assume all coils are the same — they are not. If you are replacing a coil-on-plug, you may also want to replace the spark plug at the same time, since a worn spark plug can have damaged the coil by forcing it to work too hard.
After installation, clear the check engine light using a scan tool or by disconnecting the battery for 15 minutes. Start the engine and listen for the misfire to be gone. If it is, the coil was the problem. If the misfire remains, you have a different issue — fuel delivery, compression, or another coil.
When the coil tests good but you still have a misfire
If your coil passes both the resistance test and the spark tester, but the engine still misfires, the problem is not the coil. The most common culprits are a worn spark plug, a clogged fuel injector, or low compression in that cylinder.
Start by replacing the spark plug. A plug that is worn, fouled, or gapped incorrectly can cause a misfire even if the coil is firing correctly. Plugs are cheap ($5 to $15 each) and straightforward to replace. If the misfire stops, you have found it. If it continues, have a shop run a compression test on that cylinder — low compression points to a valve or piston problem that requires engine work.
A fuel injector that is clogged or leaking can also cause a misfire on one cylinder. This is harder to diagnose at home, but a shop can test injector pulse and spray pattern with a scope. If the injector is the problem, it can usually be cleaned or replaced for $200 to $400 depending on the car.
Frequently Asked Questions
Can a bad ignition coil prevent the car from starting at all?
Yes. If the coil is completely dead, the spark plugs will not fire and the engine will not start. You will hear the starter motor turn the engine over, but it will not catch. A visual inspection or spark tester will confirm this quickly.
Do I need to replace all the coils if one is bad?
No. Replace only the coil that failed. However, if your car is older and all the coils are original, they are likely the same age and may fail soon after. Some owners replace them all at once to avoid repeat failures, but it is not required.
What if the coil looks fine but the spark tester shows no spark?
The problem could be the coil, the connector, or the wiring between them. Check that the connector is clean and clicks firmly into place. If it does, the coil is likely bad even though it looks fine — internal damage is not always visible.
Can I test the coil while it is still plugged into the car?
A spark tester can be used while the coil is installed. A multimeter resistance test must be done with the coil unplugged, because the car's computer and other components will interfere with the reading.
How long does a new coil last?
A replacement coil should last as long as the original — typically 80,000 to 150,000 miles. If it fails much sooner, the underlying problem (a bad spark plug, a fuel system issue, or engine overheating) is still present and needs to be fixed.