Checking the condition of a spark plug involves both physical inspection and a simple electrical test. The most definitive way to test its internal integrity is by using a multimeter.
A reliable method to check a spark plug's condition is by testing its internal resistance using a multimeter.
Using a Multimeter to Test Internal Resistance
This test assesses the continuity and resistance of the internal path within the spark plug, which includes the resistor (if present) and the central electrode.
Here's how to perform the test:
- Gather Equipment: You will need a multimeter capable of measuring resistance (Ω - ohms).
- Set the Multimeter: Turn your multimeter dial to the ohms (Ω) setting. Choose a range that can accommodate readings up to at least 20-50 ohms (e.g., 200 Ω scale).
- Connect Probes:
- Touch one multimeter probe to the terminal end (the top snap connector where the spark plug wire attaches).
- Touch the other multimeter probe to the top of the central electrode (the metal tip at the very end of the plug that sits inside the combustion chamber). Be careful not to touch the side electrode (the ground strap).
- Read the Measurement: Observe the reading on the multimeter display.
Interpreting the Multimeter Reading
According to technical guidelines, a specific range indicates a healthy spark plug:
- Reading Between 5 and 15 Ω: A reading between 5 and 15 ohms when touching the terminal end and the top of the central electrode indicates that your spark plug is in good shape.
- No Reading (Open Circuit): If there is no reading on the multimeter (often displayed as "OL" or a very high number indicating infinite resistance, or simply 0 depending on the meter type and range), it suggests the internal metal path or the resistor running insulated between these two points might be broken or have an open circuit.
This electrical test specifically verifies the internal components are conducting electricity properly.
Visual Inspection (Complementary Check)
While the multimeter test checks internal health, a visual inspection can reveal external issues:
- Electrode Condition: Check the central and ground electrodes for wear, corrosion, or damage. The edges should be relatively sharp, not rounded or severely eroded.
- Fouling: Look for deposits on the firing tip.
- Black, Sooty Deposits: Can indicate a rich fuel mixture or weak spark.
- White Deposits: Can suggest engine overheating or incorrect heat range plug.
- Oily Deposits: May indicate oil entering the combustion chamber.
- Ceramic Insulator: Inspect the ceramic part for cracks or chips. Damage here can cause the spark to misfire by jumping to the engine block instead of across the electrode gap.
- Gap: Check the gap between the central and ground electrodes using a spark plug gap tool. The gap should be within the manufacturer's specifications for your vehicle.
By combining a visual inspection with the multimeter test, you get a more complete picture of the spark plug's condition. The multimeter test, particularly checking for a resistance reading between 5 and 15 ohms, is a key indicator of the spark plug's internal electrical health.
| Test Method | What It Checks | Indicator of Good Condition | Indicator of Bad Condition |
|---|---|---|---|
| Multimeter (Ohms) | Internal resistance/continuity | Reading between 5 and 15 Ω | No reading (Open Circuit/Broken) |
| Visual Inspection | Physical wear, deposits, damage | Electrodes sharp, clean, no cracks | Electrodes worn, fouled, cracked |
