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Testing a Spark Plug with and without a Multimeter: Full Guide

Table of content

  1. Where is the spark plug in my car?
  2. What tools do I need to test a spark plug?
  3. Removing the spark plug
  4. Spark plug diagnosis guide
    1. Broken or flattened electrodes
    2. Burned electrodes
    3. Black or dark brown carbon deposits
    4. Black oily deposits
    5. Wet plugs
  5. How to test spark plugs using a multimeter
  6. How to test spark plugs without a multimeter
  7. Can you check spark plugs without removing them?
  8. Can a bad spark plug cause check engine light?

You may be able to determine whether a car spark plug has seen better days just by looking at it. There are often obvious signs of wear, damage or contamination, which will be outlined in the guide below. We’ll also discuss how to test a spark plug using a multimeter.

Before we dive into that, we should establish the ground rules for your safety. Before inspecting or testing the spark plugs, you should make sure to wear appropriate protective gear, such as safety glasses and protective work gloves. Do not attempt to remove the components while the engine is still hot and make sure the ignition is switched off at all times while handling the components.

Where is the spark plug in my car?

Spark plugs are located at the top of each cylinder in your engine, threaded into the cylinder head, with spark plug wires leading to the top of the engine. In newer vehicles with "coil-on-plug" designs, you'll see plastic modules directly attached to each spark plug instead of wires.

The number of spark plugs depends on the number of cylinders in your engine. On a 4-cylinder engine, you'll find the spark plugs arranged in a row along the top or side of the engine. For inline 6-cylinder engines, the spark plugs are positioned on the top or side of the engine head. In V6 and V8 engines, the spark plugs are evenly distributed on both sides of the engine, matching the V-formation of the cylinders.

What tools do I need to test a spark plug?

For a proper test, you'll need several specific tools that help diagnose the condition and performance. A thorough inspection requires both visual assessment and electrical testing.

Required tools include:

  • Spark plug socket (usually 5/8" or 13/16")
  • Ratchet and extension
  • Spark plug tester (inline ignition tester)
  • Feeler gauge for measuring electrode gap
  • Wire brush for cleaning carbon deposits
  • Inspection light or torch
  • Multimeter to check resistance
  • Compression tester to assess cylinder condition

Spark plug diagnosis guide

These tools allow you to remove, inspect, clean, and test your spark plugs to check if they're functioning correctly or need replacement. Testing spark plugs can reveal problems with your ignition system and overall engine performance.

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Removing the spark plug

Ensure that the car is parked on a hard flat surface and the engine is cool. Make sure the engine and ignition are switched off and the key is removed from the ignition before proceeding. Check the manufacturer's repair guide for specific safety instructions.

You’ll need to disconnect the spark plugs from the ignition system. Older vehicles typically have a wire connecting the plugs to the ignition, whereas newer vehicles normally have a separate ignition coil for each spark plug and use coil-on-plug connectors. Coil-on-plug connectors usually feature a locking tab that can be released using a screwdriver.

Using the correct size spark plug wrench and socket, gently unscrew the spark plug from the cylinder head and remove the component.

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LPG1 NGK LPG Laser Line 1496 Spark plug CNG/LPGM14 x 1,25, Spanner Size: 16 mm
Reviews - 79
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  • Product line: LPG Laser Line
  • Spark Plug: Fixed SAE connector
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  • Fuel Type: CNG / LPG
  • Recommended change interval [km]: 60000
  • NGK: Spark plug
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0 002 240 911 BERU ULTRA Z332 Spark plug 12 ZR-6 SPP2-1, M12x1,25, Spanner Size: 14 mm Bi-Hex
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Parts Details
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  • Tightening torque from [Nm]: 15
  • Tightening torque to [Nm]: 25
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DIFR5C NGK 90911 Spark plug M14 x 1,25, Spanner Size: 16 mm
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  • Spark Plug: Iridium Middle Electrode, 1 - Earthed Electrode, Interference Suppression 5 kOhm, with gasket seat, Fixed SAE connector
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  • NGK: Spark plug
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ILFER7C8EG NGK 91898 Spark plug M14 x 1,25, Spanner Size: 16 mm
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Parts Details
  • Spark Plug: Ring type connector (cup-shaped), Iridium Middle Electrode, 1 - Earthed Electrode, Interference Suppression 1 kOhm, with gasket seat
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  • Spark Position [mm]: 3,5
  • Thread Size: M14 x 1,25
  • NGK: Spark plug
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Details
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compared to RRP £ 19,16

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Spark plug diagnosis guide

Look for the following signs once the plugs have been removed:

Broken or flattened electrodes

If the ground or centre electrode is broken or the ground electrode has been flattened, shortening the gap between the two, the components will not be able to produce a normal spark. This can occur when the wrong size spark plug is installed.

Burned electrodes

Melting or white deposits are a sign of thermal damage caused by engine overheating, an unbalanced air-fuel mixture, timing faults or an incorrect heat range.

Black or dark brown carbon deposits

Some deposits can be expected but if the plugs are unusually dark and sooty, they could be over contaminated. Carbon fouling is often the result of a bad air filter, a rich air-fuel mixture, or poor driving habits.

Black oily deposits

This could indicate that oil is leaking into the engine cylinder head.

Wet plugs

This is often the result of a flooded engine.

While most spark plugs work effectively throughout their service life provided that there are no underlying issues, it is still recommended to only use parts from trusted brands, such as NGK spark plugs, to ensure long-lasting efficiency.

How to test spark plugs using a multimeter

You can test your spark plugs using a multimeter. Simply set the device to the 20K ohms setting and place the probes on both ends of the plug; one on the terminal end and one at the tip of the centre electrode.

You should see a reading between 5 and 15. If there’s no reading, the internal components may be damaged. If there’s a reading when one probe touches the terminal end and the other touches any other metal part of the plug, this indicates a malfunction.

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  • Tightening torque from [Nm]: 25, 20
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  • Resistor from [kOhm]: 1
  • Resistor to [kOhm]: 3
  • BERU: Spark plug
  • Item number: Z90
  • Condition: New
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BERU Spark plug M14x1.25, Spanner Size: 16mm
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5650 DENSO Iridium Tough VXUH20I Spark plug M12x1.25,
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Parts Details
  • Product line: Iridium Tough
  • Spark Plug: with resistor, 1 - Earthed Electrode, Iridium Middle Electrode, Platinum mass electrode
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  • DENSO: Spark plug
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DENSO Spark plug M12x1.25,
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+40 BOSCH 0 242 129 510 Spark plug M 12 x 1,25, Spanner Size: 14
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VAR6NIP BOSCH 0 242 140 565 Spark plug M 12 x 1,25, Spanner Size: 14
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  • Electrode Gap [mm]: 0,7
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  • Number of pins: 1
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DIFR6D13 NGK 94167 Spark plug M14 x 1,25, Spanner Size: 16 mm
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Parts Details
  • Spark Plug: Iridium Middle Electrode, 1 - Earthed Electrode, Interference Suppression 5 kOhm, with gasket seat, Fixed SAE connector
  • Spanner Size: 16 mm
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  • NGK: Spark plug
  • Item number: 94167
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NGK Spark plug M14 x 1,25, Spanner Size: 16 mm
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How to test spark plugs without a multimeter

Testing spark plugs without a multimeter is straightforward using visual inspection and simple tools. First, remove the spark plug using a socket wrench with the proper size socket. Examine the plug's appearance – a healthy plug should have a light brown or greyish electrode with minimal deposits. Black, sooty deposits indicate a rich fuel mixture, while white, chalky deposits suggest the engine is running too hot.

To check for spark, use an inline spark tester connected between the ignition coil and spark plug. Alternatively, you can remove the plug, reconnect it to the ignition cable, ground the threaded portion against the engine block, and have someone crank the engine while you watch for a visible spark across the electrode gap.

Listen for regular clicking sounds during cranking, which indicates the ignition system is firing properly.

Can you check spark plugs without removing them?

You can perform limited spark plug checks without removal. Listen for misfires or rough idling, which may indicate plug issues. Use an engine diagnostic scanner to detect misfire codes. For a more direct test, use an inline spark tester connected between the ignition coil and plug – a visible spark confirms the ignition system is working.

However, important assessments like examining electrode wear, measuring gap width, and checking for deposits require removing the plugs. Without removal, you can only infer potential problems rather than confirming specific issues with the spark plugs themselves.

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Can a bad spark plug cause check engine light?

A bad spark plug can certainly trigger the check engine light. When spark plugs fail, they cause engine misfires that disrupt the combustion process. Modern vehicles have sensors that detect these misfires, and when they occur repeatedly, the engine control module records the issue and illuminates the check engine light.

Common diagnostic trouble codes related to spark plug problems include P0300 (random/multiple cylinder misfire) or specific cylinder misfire codes like P0301 (cylinder 1 misfire). These codes alert you to investigate the ignition system, with faulty spark plugs being a primary suspect. Replacing worn or damaged spark plugs often resolves these check engine light warnings.

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