How to Test an Ignition Coil & Spark Plug

Written by sam orr
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How to Test an Ignition Coil & Spark Plug
Visually inspect spark plugs for signs of damage or fouling. (old spark plugs image by Rog999 from

Engine misfiring, increased fuel consumption, and overall performance loss can often be attributed to faulty spark plugs or ignition coils. A single fouled plug will decrease a four-cylinder engine's power output by up to 25 per cent. Identify the point of spark loss for your vehicle and you will gain vital information for diagnosing and correcting underlying engine issues in the process. Because ignition systems vary greatly in design, familiarise yourself with the basic ignition design for your vehicle before you begin troubleshooting procedures and repairs. Consult your vehicle service information or repair manual for diagrams, manufacturer specifications and specific testing procedures.

Skill level:

Things you need

  • Insulated spark plug boot pliers
  • Spark plug ratchet/socket, 3/8"
  • Ratchet extension
  • Impedance Ohmmeter (10 Megaohm)
  • Vehicle Service or Repair Guide
  • Safety goggles and gloves
  • An assistant

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  1. 1

    Turn the car engine off and allow it to cool completely before removing any spark plugs to avoid damaging aluminium threads on the cylinder heads.

  2. 2

    Grasp the spark plug wire by the insulated boot connector at the end of the spark plug. Use insulated boot pliers designed to maximise access to recessed plugs and avoid wire damage. Apply a left-to-right twisting motion to loosen the boot seal and pull outward to remove the plug wire and boot from the plug.

  3. 3

    Align the 3/8 inch plug ratchet over the exposed spark plug and turn counterclockwise to loosen. Finish unscrewing the plug by hand and remove the plug. Reconnect the plug to the plug wire at the boot using a twisting motion until the boot "clicks" into contact with the plug. Place this plug and wire on an exposed metal grounding surface.

    How to Test an Ignition Coil & Spark Plug
    A plug ratchet or socket wrench is necessary for removing tight spark plugs. (sparkplug wrench image by Aleksandr Ugorenkov from
  4. 4

    Instruct your assistant to start the engine. Stand clear of the vehicle and observe the spark plug gap (between the ground and centre plug electrode) for a strong and bright spark, indicating that the plug and wire are functional. Shut off the car's engine.

  5. 5

    Disconnect the wire and boot from the plug. Replace the plug by hand, using the plug ratchet to tighten the plug. Reconnect the wire and boot and test the remaining plugs, removing and replacing plugs one at a time.

    How to Test an Ignition Coil & Spark Plug
    Faulty spark plug wires can leak current and short the spark to the spark plug or increase the resistance. (car engine image by itsallgood from
  6. 6

    Use an ohmmeter to test the plug wire of any malfunctioning units. Disconnect the plug wire from the plug at one end and from the distributor cap at the other end. Measure the wire's resistance by connecting an ohmmeter test lead to each end of the wire.

  7. 7

    Compare this reading to the resistance specifications reported in the vehicle service manual. Replace the entire set of plug wires if more than one wire measurement is outside of the specified resistance range.

  1. 1

    Disconnect all lead wires to the ignition coil to isolate the coil from the battery, distributor and module. Remove leads on "single can" coil systems that are attached to the top of the coil at the high voltage output terminal (in the centre of the ignition coil) and also at the positive and negative poles or terminals located at either side of this central terminal. Consult manufacturer specifications in your vehicle's service manual for model-specific instructions.

  2. 2

    Connect the ohmmeter test leads to the positive and negative coil terminals. For enclosed units and systems without distributors (DIS), refer to your vehicle service information for terminal locations if the terminal poles are not indicated or marked.

    How to Test an Ignition Coil & Spark Plug
    Resistance readings are reported in ohm units. (polimetro image by ANTEQUERANUS from
  3. 3

    Read and record the primary resistance measurement found on the ohmmeter display.

  4. 4

    Disconnect the leads from the primary terminals. Connect the ohmmeter leads to the positive terminal and to the high voltage output terminal at the centre of the coil. Read and record this secondary coil resistance measurement.

  5. 5

    Compare these primary and secondary resistance values to coil test resistance specifications provided in the manufacturer's service guide information. Readings outside of the recommended range for your vehicle can indicate faulty coils.

Tips and warnings

  • Traditional distributor-based engine systems normally contain a "single can" ignition coil, although some imports feature dual coil systems. Those without distributors (DIS) and "waste spark" setups will vary in design and may feature enclosed coil units, multiple coils or "coil-on-plug" (COP) setups.
  • Terminal locations will vary but are often indicated or marked on the coil unit for newer enclosed coil units and systems without distributors. Terminals may be found beneath the coils or within a connector for these systems, so consult your vehicle service information or repair manual for terminal locations and specific testing procedures for coils that are not a "single can" style.
  • Adjust the ohmmeter to take low range readings, if necessary, in order to measure primary coil resistance. These resistances are on the order of two ohms or less. Secondary resistance measurements are much higher and fall between 6,000 to 30,000 ohms.
  • Ignition coils act as "high voltage transformers," increasing the twelve volt battery output to the thousands of volts needed to power the ignition system. Secondary ignition coils produce these very high voltages, ranging from 4,000 to 28,000 volts, and a spark plug wire carries this ignition voltage. Holding this wire while the engine is running may be a common practice when testing for a spark, but is dangerous and is not recommended

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