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How to test a spark plug

Table of content

  1. Removing the spark plug
  2. Spark plug diagnosis guide
  3. How to test spark plugs

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.

Removing the spark plug

  1. 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.
  2. 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.
  3. Using the correct size spark plug wrench and socket, gently unscrew the spark plug from the cylinder head and remove the component.

ILZKBR7B8DG NGK Bi-Hex 95770 Spark plug M12 x 1,25, Spanner Size: 14 mm
Parts Details
  • Spanner Size: 14 mm
  • Spark Plug: Iridium Middle Electrode, 1 - Earthed Electrode, Interference Suppression 5 kOhm, with gasket seat
  • Thread Length [mm]: 26,5
  • Spark Position [mm]: 5,0
  • Thread Size: M12 x 1,25
  • Version: Bi-Hex
  • NGK: Spark plug
  • Item number: 95770
  • Condition: New
Details
NGK Spark plug M12 x 1,25, Spanner Size: 14 mm
Article number: 95770
£ 13,66
(Price per item)
price incl. 20% VAT, excl. delivery costs
4604 DENSO Nickel TT K20TT Spark plug Spanner Size: 16
Parts Details
  • Spanner Size: 16
  • Product line: Nickel TT
  • Spark Plug: 1 - Earthed Electrode, Nickel Middle Electrode
  • Thread Length [mm]: 19
  • Connector type: Solid SAE
  • Tightening Torque [Nm]: 22
  • Diameter [mm]: 14
  • Weight [g]: 43
  • for OE number: 1WT318110
  • Gross Weight [g]: 46
  • DENSO: Spark plug
  • Item number: K20TT
  • Condition: New
Details
DENSO Spark plug Spanner Size: 16
Article number: K20TT
£ 3,66
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5346 DENSO Iridium Power IKH24 Spark plug Spanner Size: 16
Parts Details
  • Spanner Size: 16
  • Product line: Iridium Power
  • Spark Plug: 1 - Earthed Electrode
  • Thread Length [mm]: 26,5
  • Connector type: Crimped SAE
  • Tightening Torque [Nm]: 22
  • Diameter [mm]: 14
  • Weight [g]: 48
  • for OE number: 999 170 232 90
  • Country Specific Design: Japan
  • Gross Weight [g]: 51
  • DENSO: Spark plug
  • Item number: IKH24
  • Condition: New
Details
DENSO Spark plug Spanner Size: 16
Article number: IKH24
£ 18,90
(Price per item)
price incl. 20% VAT, excl. delivery costs
5340 DENSO Iridium Power ITV22 Spark plug Spanner Size: 16
Parts Details
  • Spanner Size: 16
  • Product line: Iridium Power
  • Spark Plug: 1 - Earthed Electrode
  • Thread Length [mm]: 25
  • Connector type: Crimped SAE
  • Tightening Torque [Nm]: 22
  • Diameter [mm]: 14
  • Weight [g]: 47
  • for OE number: 1698597
  • Country Specific Design: Japan
  • Gross Weight [g]: 50
  • DENSO: Spark plug
  • Item number: ITV22
  • Condition: New
Details
DENSO Spark plug Spanner Size: 16
Article number: ITV22
£ 12,39
(Price per item)
price incl. 20% VAT, excl. delivery costs
ILNAR8B7G NGK 91970 Spark plug M12 x 1,25, Spanner Size: 14 mm
Parts Details
  • Spanner Size: 14 mm
  • Spark Plug: Iridium Middle Electrode, 1 - Earthed Electrode, Interference Suppression 5 kOhm, with taper gasket seat, Fixed SAE connector
  • Thread Length [mm]: 25,0
  • Spark Position [mm]: 3,5
  • Thread Size: M12 x 1,25
  • NGK: Spark plug
  • Item number: 91970
  • Condition: New
Details
NGK Spark plug M12 x 1,25, Spanner Size: 14 mm
Article number: 91970
£ 20,54
(Price per item)
price incl. 20% VAT, excl. delivery costs

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.

Spark plug diagnosis guide

  • 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

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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5310 DENSO Iridium Power IK22 Spark plug Spanner Size: 16
Parts Details
  • Spanner Size: 16
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  • Spark Plug: 1 - Earthed Electrode
  • Thread Length [mm]: 19
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R7282-11 NGK 2001 Spark plug M14 x 1,25, Spanner Size: 16 mm
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3417 DENSO Iridium SK16HR11 Spark plug Spanner Size: 16
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3426 DENSO Super Ignition Plug FK20HR11 Spark plug Spanner Size: 16
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PFR6Q NGK 6458 Spark plug M14 x 1,25, Spanner Size: 16 mm
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PFR6W-TG NGK 5547 Spark plug M14 x 1,25, Spanner Size: 16 mm
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PTR6F-13 NGK 7569 Spark plug M14 x 1,25, Spanner Size: 16 mm
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LPG6 NGK LPG Laser Line 1565 Spark plug CNG/LPGM14 x 1,25, Spanner Size: 16 mm
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  • Spanner Size: 16 mm
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  • Spark Plug: Iridium Middle Electrode, Fixed SAE connector
  • Thread Length [mm]: 19,0
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  • Recommended change interval [km]: 60000
  • NGK: Spark plug
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BKR8EIX NGK 2668 Spark plug M14 x 1,25, Spanner Size: 16 mm
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LZKR6AI-10G NGK 97999 Spark plug M12 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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  • Thread Size: M12 x 1,25
  • NGK: Spark plug
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T08 DENSO Nickel TT XU22TT Spark plug Spanner Size: 16
Parts Details
  • Spanner Size: 16
  • Product line: Nickel TT
  • Spark Plug: 1 - Earthed Electrode, Nickel Middle Electrode
  • Thread Length [mm]: 19
  • Connector type: Solid SAE
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  • Diameter [mm]: 12
  • Weight [g]: 38
  • for OE number: 1WT418110
  • Country Specific Design: Japan
  • Gross Weight [g]: 41
  • DENSO: Spark plug
  • Item number: XU22TT
  • Condition: New
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DENSO Spark plug Spanner Size: 16
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DIFR6D13 NGK 94167 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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  • Thread Size: M14 x 1,25
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5317 DENSO Iridium Power IW27 Spark plug Spanner Size: 20.6
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5610 DENSO Iridium Tough VK22 Spark plug Spanner Size: 16
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4713 DENSO Iridium TT IT16TT Spark plug Spanner Size: 16
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