What a multimeter can and cannot tell you about a spark plug
A multimeter measures electrical resistance and voltage, which means it can tell you whether a spark plug's central electrode is broken or whether the ceramic insulator has cracked all the way through. It cannot tell you whether the plug will actually fire inside your engine, whether the gap is correct, or whether carbon buildup is preventing a good spark. A multimeter is a first-pass screening tool — it answers "is this plug obviously dead?" but not "is this plug working well?"
If your multimeter shows the spark plug has infinite resistance (the needle does not move, or the display shows "OL"), the plug is bad and needs replacement. If it shows low or normal resistance, the plug may still be worn or fouled, and a visual inspection under magnification is the next step. Many mechanics skip the multimeter entirely and go straight to visual inspection, since what you see — electrode erosion, gap width, color of deposits — tells you more than resistance alone.
Key Takeaways
- Set your multimeter to the resistance setting (ohms), remove the spark plug from the engine, and touch one probe to the center electrode and one to the threaded shell to measure resistance across the ceramic.
- Infinite resistance (no needle movement or "OL" display) means the ceramic is cracked and the plug must be replaced.
- Low or normal resistance (typically under 10,000 ohms) means the ceramic is intact, but does not confirm the plug will fire or that the gap is correct.
- A visual inspection of the electrode tip and ceramic color after testing gives you more information than the multimeter reading alone.
- If you cannot remove the spark plug safely or your multimeter does not have an ohms setting, take the plug to a parts store or mechanic for inspection.
Gather your tools and remove the spark plug
You will need a multimeter with an ohms (resistance) setting, a spark plug socket, a ratchet wrench, and a way to safely remove the spark plug from your engine. Most multimeters sold at hardware stores have an ohms setting — look for a dial with an omega symbol (Ω) or the word "ohms". If your multimeter only has voltage and amperage settings, it cannot test spark plug resistance.
Let your engine cool completely before touching anything. Remove the spark plug wire or coil pack by twisting gently and pulling straight back — do not yank on the wire itself. Use the spark plug socket and ratchet to unscrew the plug from the cylinder head. Keep track of which cylinder it came from if you are testing multiple plugs. Place the removed plug on a clean surface where you can see it clearly.
Set the multimeter to ohms and prepare the probes
Turn the multimeter dial to the ohms setting. Most multimeters have a range of ohm settings (like 200Ω, 2kΩ, 20kΩ, 200kΩ). Start with the 20kΩ setting — this range works for most spark plugs. If the display shows "OL" (overload) or the needle does not move at all, try the 200kΩ setting. If it shows a number, that is your resistance reading.
Insert the black probe into the COM (common) jack and the red probe into the V/Ω (voltage/ohms) jack. These are the standard probe positions for resistance testing on nearly all multimeters. Make sure both probes are clean and make good contact with the metal parts of the spark plug — corrosion or loose contact will give you a false reading.
Touch the probes to the spark plug electrodes
Hold the spark plug steady with one hand. Touch the red probe to the center electrode (the metal rod in the middle of the ceramic tip). Touch the black probe to the threaded metal shell of the plug body. Press firmly so both probes make solid contact with bare metal — do not let the probes touch the ceramic insulator itself.
Hold the probes in place for two to three seconds and read the display. Write down the number you see. If you are testing multiple plugs, test each one and record the readings so you can compare them. A spark plug that has been in the engine for a long time may show slightly higher resistance than a new one, but all plugs from the same engine should read within a similar range.
Interpret the multimeter reading
A reading of infinite resistance — shown as "OL" on a digital multimeter or no needle movement on an analog meter — means the ceramic insulator is cracked or broken. The electrical path is blocked, and the plug cannot fire. Replace it.
A reading between 0 and 10,000 ohms (0 to 10kΩ) is normal for most spark plugs in good condition. Older plugs or plugs with heavy carbon buildup may read slightly higher, up to 20,000 ohms, but still function. If all your plugs read in this range, the multimeter test is inconclusive — you need to look at the plug visually to know whether it is actually worn out.
A reading of more than 20,000 ohms suggests the ceramic is degraded or heavily fouled. This does not always mean the plug is dead, but it is a sign that replacement should be considered, especially if the plug is more than three years old or has high mileage.
Inspect the spark plug visually after testing
Once you have the multimeter reading, look at the spark plug under good light or with a magnifying glass. The center electrode should have a sharp edge and a small gap (usually 0.028 to 0.040 inches, depending on your engine). If the electrode is rounded, eroded, or has a gap wider than your engine manual specifies, the plug should be replaced even if the multimeter reading was normal.
The ceramic insulator should be white or light tan. If it is black (carbon fouling), wet (oil fouling), or has a white crusty coating (ash buildup), the plug is not firing cleanly and should be replaced. A cracked or chipped ceramic is visible to the eye and means the plug must go, regardless of the multimeter reading.
If the multimeter showed infinite resistance and the ceramic looks intact, retest — you may not have made solid contact with the probes the first time. If it still shows infinite resistance, the crack is internal and not visible, and the plug needs replacement.
What to do if the test is unclear
If your multimeter reading is borderline (between 10,000 and 20,000 ohms) and the plug looks worn but not obviously bad, you have two options. You can replace the plug as preventive maintenance — spark plugs are inexpensive compared to the cost of a misfire or engine damage. Or you can reinstall the plug, drive the vehicle, and monitor for rough idle, hesitation, or poor fuel economy, which are signs of a failing spark plug.
If you do not have a multimeter or are not comfortable testing the plug yourself, take it to an auto parts store or a mechanic. Many parts stores will inspect a spark plug for free and tell you whether it needs replacement. A mechanic can also check the gap with a gap tool and inspect the ceramic under magnification, which gives you more information than a multimeter alone.
Frequently Asked Questions
Can I test a spark plug while it is still in the engine?
No. The multimeter needs direct contact with the center electrode and the metal shell, and you cannot reach both safely while the plug is installed. You must remove it first. Testing while installed also risks damaging the multimeter if you accidentally touch the engine block or another conductor.
What if all my spark plugs show different resistance readings?
Some variation is normal — plugs wear at different rates depending on which cylinder they are in and how the engine runs. If one plug reads infinite resistance and the others read normal, replace that one. If all of them read high or show infinite resistance, all four (or six, or eight) plugs are likely due for replacement.
Does a normal multimeter reading mean the spark plug is good?
Not necessarily. A normal resistance reading means the ceramic is not cracked, but it does not tell you whether the electrode is eroded, the gap is wrong, or the plug is fouled with carbon or oil. You still need to look at the plug visually and compare the gap to your engine manual's specification.
Can I reuse a spark plug if the multimeter shows it is okay?
Only if the visual inspection also looks good — sharp electrode, correct gap, clean ceramic, and less than three years old or under 30,000 miles. If the electrode is rounded or eroded, or if the ceramic is discolored, replace the plug even if the multimeter reading is normal. Worn plugs cause misfires and poor fuel economy.
What if my multimeter shows 0 ohms?
A reading of exactly 0 ohms means there is a direct short between the center electrode and the shell — the ceramic is completely gone or the plug is damaged. This is a dead plug and must be replaced. Make sure the probes are not touching each other, which would also show 0 ohms.