What a multimeter tells you about an ignition coil
A multimeter measures electrical resistance and voltage, which are the two things that fail in an ignition coil. When you test with a multimeter, you are checking whether the coil's internal windings are intact and whether it can hold an electrical charge. A coil with broken windings will show infinite resistance (displayed as "OL" on most multimeters). A coil that is leaking voltage to ground will show resistance that is too low. Neither condition means the coil is definitely bad — a multimeter test is one piece of information, not a diagnosis — but it narrows down whether the coil itself is the problem or whether something else in the ignition system is causing your symptoms.
The test takes about five minutes per coil and requires only a multimeter set to the resistance setting (ohms). You do not need to remove the coil from the engine, though you do need to disconnect it from the wiring harness and let the engine cool if it has been running.
Key Takeaways
- Set your multimeter to the ohms (resistance) setting before touching the coil terminals, and make sure the engine is off and cool.
- Test between the two primary terminals (the ones the spark plug wires or coil pack connector plugs into) to check the primary winding resistance.
- Test between a primary terminal and the secondary terminal (the spark plug wire connection) to check the secondary winding resistance.
- Write down the resistance readings and compare them to your vehicle's service manual, because acceptable ranges vary widely by coil type and engine.
- A reading of infinite resistance (OL on the display) or zero resistance usually means the coil has failed, but a normal reading does not rule out other ignition problems.
Disconnect the coil and let it cool
Turn off the engine and wait at least ten minutes before touching the coil. Ignition coils get hot during operation, and you can burn your hand on a warm coil. A cool coil also gives you a more accurate resistance reading because resistance changes slightly with temperature.
Locate the coil you want to test. On older vehicles, there is usually one coil mounted on or near the distributor. On newer vehicles, there is often one coil per spark plug, mounted directly on top of the cylinder head. Look for a cylindrical or rectangular component with a thick wire or connector coming out of it.
Disconnect the wiring harness from the coil by squeezing or unclipping the connector. Do not pull on the wires themselves — pull on the plastic connector body. If the coil is held in place by a bolt or clip, you do not need to remove it; you only need to disconnect the electrical connector.
Set the multimeter to ohms and prepare to test
Turn the multimeter dial to the ohms setting. On most multimeters, this is marked with the Greek letter omega (Ω). If your multimeter has multiple ohm ranges (like 200Ω, 2000Ω, 20kΩ), start with the highest range. You can switch to a lower range if the reading is too small to read clearly.
Look at the coil terminals. An older single coil usually has two large primary terminals where the spark plug wires connect, and sometimes a third smaller terminal. A coil pack (the kind mounted on the spark plugs) has a connector with multiple pins. You need to identify which terminals are which. The service manual for your vehicle will show you, but in general, the two thick terminals are the primary windings, and any other connection is secondary.
Before you touch the probes to the coil, make sure your hands are dry and you are not standing on a wet surface. Static electricity can damage some coils, though most are hardy enough that this is not a major concern. It is still good practice.
Test the primary winding resistance
Touch one multimeter probe to one primary terminal and the other probe to the second primary terminal. Hold them steady for two or three seconds. The multimeter will display a number. Write this number down.
Primary winding resistance on most coils ranges from 0.4 ohms to 2 ohms, but this varies. Some coils read as low as 0.1 ohms, and some read as high as 5 ohms. Do not assume your reading is wrong just because it does not match another coil you tested — different coil types have different specifications.
If the multimeter displays "OL" (open loop, meaning infinite resistance), the primary winding is broken and the coil has failed. If it displays "0" or very close to zero, the winding may be shorted. Either result suggests the coil should be replaced, but check your manual first to confirm the expected range for your specific coil.
Test the secondary winding resistance
Touch one probe to a primary terminal and the other probe to the secondary terminal (the one where the spark plug wire connects, or the center terminal on a coil pack). Hold steady for two or three seconds and write down the reading.
Secondary winding resistance is much higher than primary resistance because the secondary winding has many more coils of wire. Most secondary readings fall between 4,000 and 10,000 ohms (4kΩ to 10kΩ), but again, your manual will specify the correct range for your coil. Some coils read as high as 15,000 ohms.
If you see "OL" on the secondary test, the secondary winding is open and the coil cannot produce spark. If you see a very low number (under 1,000 ohms), the secondary winding may be shorted. Either result points to coil failure.
Compare your readings to the service manual
Look up the resistance specifications for your coil in the service manual or the manufacturer's data sheet. You can often find this information online by searching for your vehicle year, make, model, and "ignition coil resistance specifications." Some repair websites and forums also list common coil specs.
If your primary reading falls within the specified range and your secondary reading falls within the specified range, the coil's internal windings are intact. This does not mean the coil is working perfectly — a coil can fail in ways a resistance test cannot detect, such as breaking down under high voltage — but it does mean the basic structure is sound.
If either reading is outside the range, or if you got "OL" or zero on either test, the coil is likely defective. Before you replace it, however, make sure you tested the right terminals. If you tested the wrong pair of terminals, your reading will be meaningless.
Reconnect the coil and consider what the test means
Plug the wiring harness back into the coil connector. Make sure it clicks or seats fully. A loose connection will cause the same symptoms as a bad coil — misfiring, rough idle, or no start — so this step matters.
If your resistance readings were normal, the coil itself is probably not the problem. Your symptoms may be caused by a bad spark plug, a cracked spark plug wire, a failing ignition module, a weak battery, or a fuel system issue. If your readings were abnormal, the coil should be replaced.
A multimeter test is useful because it rules out one component quickly and cheaply. It is not a complete diagnosis of the ignition system. If you replace the coil and the problem persists, you will need to test other parts of the ignition circuit or have the vehicle scanned for fault codes.
Frequently Asked Questions
Can I test the coil while it is still plugged in?
No. The multimeter will read the resistance of the entire circuit, not just the coil. You must disconnect the coil from the wiring harness first. Testing a connected coil will give you a false reading.
What does "OL" mean on my multimeter?
OL stands for open loop, which means the multimeter is reading infinite resistance. This happens when there is a break in the circuit — in this case, a broken winding inside the coil. A coil that reads OL on either the primary or secondary test has failed and needs replacement.
My coil resistance is slightly outside the manual range. Does that mean it is bad?
Not necessarily. Resistance changes slightly with temperature, and old coils naturally drift a bit over time. If your reading is within 10 to 15 percent of the specified range, the coil is probably still functional. If it is far outside the range or reads OL, replacement is the safer choice.
Do I need to test every coil on my vehicle?
Only if you have symptoms on multiple cylinders or if you want to rule out the coils as a cause of your problem. If your vehicle is misfiring on one cylinder only, test the coil for that cylinder first. If it reads normal, test the spark plug and plug wire for that cylinder next.
What if the multimeter probe does not fit the coil terminal?
Some coil terminals are recessed or have unusual shapes. You can buy multimeter probe extensions or needle-point probes that fit into tight spaces. Alternatively, you can carefully insert a small piece of wire into the terminal and touch the multimeter probe to the wire. Make sure the wire makes solid contact with the terminal.