What an ignition coil does and why it fails
An ignition coil converts your battery's 12-volt current into the 20,000+ volts needed to fire your spark plugs. When it fails, your engine won't start, misfires under load, or stalls without warning. You can test one yourself with a multimeter and a few minutes of work — you don't need a shop to tell you whether it's dead or still working.
Ignition coils fail because of heat, moisture, and age. They sit near the engine where temperatures swing wildly, and water from rain or washing can seep into the coil pack. Most last 80,000 to 150,000 miles, but a bad one can go out at 40,000 or run past 200,000. Testing tells you whether the coil itself is the problem or whether you're chasing something else.
Key Takeaways
- A multimeter set to ohms (resistance) can tell you if a coil is dead or working, and costs $15 to $30.
- You need the coil's resistance specs from your vehicle's service manual or the coil's label — every coil has different numbers.
- A reading of infinity or zero ohms means the coil is bad; a reading within spec means it's probably fine.
- Visual inspection for cracks, corrosion, or oil leaks around the coil can catch problems before you test.
- If the coil tests good but your engine still misfires, the problem is likely the spark plug, wire, or fuel system, not the coil.
Gather your tools and locate the coil
You need a multimeter (a device that measures electrical resistance, voltage, and current). A basic one runs $15 to $30 at any auto parts store or online. You'll also need your vehicle's service manual or owner's manual — this tells you what resistance your coil should read. Many coils also have the spec printed on the coil itself.
Find the ignition coil by opening your hood and looking for a black plastic pack sitting on top of or near the engine. On older vehicles, there's usually one coil. On newer ones, there's often one coil per spark plug, mounted directly on top of each plug. If you can't spot it, your service manual has a diagram showing exactly where it sits.
Let the engine cool for at least 30 minutes before you touch anything. Ignition coils run hot, and you can burn yourself on a warm one. Once it's cool, you can safely disconnect the coil's electrical connector and remove it if needed.
Disconnect the coil and set up your multimeter
Locate the electrical connector on the coil — it's usually a plastic plug with two or four wires running into it. Gently pull the connector straight out. If it's stuck, wiggle it side to side while pulling; don't yank hard or you'll break the connector.
Set your multimeter to the ohms setting (usually marked with the Greek letter Ω). If your multimeter has a range selector, start with the lowest ohms range — often 200 ohms or less. Some multimeters auto-range, so you may not need to choose.
Look at the coil's label or your service manual and write down the resistance spec. It will be a range, like "0.4 to 0.6 ohms" or "5 to 10 ohms" — different coils have completely different specs, so you must have the right number for your coil.
Test the coil's primary winding resistance
The primary winding is the low-voltage side of the coil — the part that connects to your battery. This is what you test first. Touch one multimeter probe to one terminal inside the coil connector and the other probe to the second terminal. The order doesn't matter for resistance testing.
Read the number on the multimeter screen. If it shows infinity (∞) or "OL" (overload), the coil's primary winding is open — it's broken and needs replacement. If it shows zero or very close to zero, the winding is shorted and the coil is also bad. If the reading falls within the spec range printed on the coil or in your manual, the primary winding is working.
Write down the reading. If it's outside the spec range but not infinity or zero, the coil is marginal — it may work now but could fail soon. Many shops would replace it at this point to avoid a breakdown later.
Test the coil's secondary winding resistance
The secondary winding is the high-voltage side — the part that sends power to the spark plug. On a coil pack, this is usually a thick rubber boot or tower sticking out of the top. On an older single coil, it's the wire that runs to the distributor.
Touch one multimeter probe to the secondary terminal (the spark plug wire connection or boot) and the other probe to one of the primary terminals. Again, the order doesn't matter. Read the resistance on the screen.
Secondary resistance is much higher than primary — often 5,000 to 15,000 ohms or more, depending on the coil. Check your manual or the coil label for the exact spec. If you see infinity or zero, the secondary winding is bad. If the reading is way outside the spec range, the coil is failing. If it's within range, that side is working.
Look for physical damage while you're testing
While the coil is disconnected, inspect it for cracks in the plastic housing, white or green corrosion on the terminals, or oil leaking from the base. Any of these means the coil should be replaced, even if it tests within spec — moisture or contamination will cause it to fail soon.
Check the connector itself for bent pins, corrosion, or loose wires. A corroded connector can make a good coil look bad when you test it, because the connection is poor. If you see corrosion, clean the terminals with a wire brush or fine sandpaper, then test again.
Look at the area around the coil for oil or coolant leaks from the engine. If the coil is sitting in a puddle of oil, that's why it's failing — fix the leak when you replace the coil, or the new one will fail too.
What your test results mean and what to do next
If both primary and secondary readings are within spec and you see no physical damage, the coil is working. If your engine is still misfiring or won't start, the problem is elsewhere — check the spark plugs, spark plug wires, fuel pressure, or ignition timing.
If the primary or secondary reading is outside spec, infinity, or zero, the coil is bad and needs replacement. Buy a replacement coil for your exact year, make, and model — coils are not interchangeable. Swap it in by reversing the steps: disconnect the old coil, remove any bolts holding it down, install the new one, and reconnect the electrical connector.
If the coil tests good but you're still not confident, you can also take it to a shop and ask them to test it on their equipment — many will do this for free or a small fee. This gives you a second opinion before you spend money on a replacement you might not need.
Frequently Asked Questions
Can I test an ignition coil while it's still connected to the car?
You can, but disconnecting it first is safer and gives you a cleaner reading. If the coil is connected, you're testing through the entire circuit, and other components can affect the reading. Disconnecting takes 30 seconds and removes that noise.
What if my multimeter shows a reading that's close to spec but not exactly on it?
Multimeters have a margin of error, usually 2 to 5 percent. If your reading is within 5 percent of the spec range, the coil is fine. If it's 10 or 20 percent off, the coil is weakening and may fail soon — many technicians replace coils at this point rather than wait for a breakdown.
Do I need to remove the coil from the car to test it?
No. You can test it in place by disconnecting the connector and probing the terminals. Removing it is only necessary if you want to inspect it closely for cracks or corrosion, or if the connector is too tight to reach comfortably.
My coil tested good, but my engine still misfires. What else could it be?
Check the spark plugs first — they wear out and can cause misfires even if the coil is fine. Then check spark plug wires or coil boots for cracks or loose connections. If those are good, the problem could be fuel pressure, a clogged fuel injector, or a vacuum leak. A shop can test these with a fuel pressure gauge or smoke machine.
How often should I test my ignition coil?
You don't need to test it on a schedule. Test it only when your engine won't start, misfires, or stalls. If it tests good and you're not having problems, leave it alone. Coils fail when they fail — there's no way to predict it by testing a working one.