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How To Diagnose And Fix A Spark Plug Under 30 Mins


Spark Plug Diagnosis

The role of a spark plug in a gasoline engine is to provide the spark necessary for combusting the air-fuel mixture in the engine. This spark is achieved by creating a voltage difference between the electrodes of the spark plug. These electrodes sit inside the combustion chamber of the engine and ignite the compressed fuel and air mixture for power.


A spark plug is not an expensive item to be replaced, but if you have a habit of replacing your parts before checking them, you will eventually end up with hefty replacement costs. Problems like misfires, poor fuel economy, starting troubles, and rough engine idling can be caused due to various issues in an engine. The simplest way to troubleshoot such an issue is to start by checking the spark plugs first. Often, a visual inspection is enough to determine a faulty plug, but a proper diagnosis is necessary to ensure if the spark plug is actually firing. The following tests are very simple and will help you determine whether your spark plugs are faulty and the source of your problems or not.


Tools Required To Test Your Spark Plugs

How To Test A Faulty Spark Plug?

Using A Multimeter (Under 30 Minutes)

Step 1: Test The Resistance Of The Spark Plug

  • Remove the spark plugs from the cylinder heads using a spark plug socket wrench and set the multimeter to 20k or 20,000 ohms.
  • Touch the one probe of the multimeter to the spark plug terminal and the other end to the central electrode.
  • A properly functioning spark plug should give a reading of 4,000 to 8,000 ohms on the multimeter. 
  • If the multimeter shows readings below this range, then your plugs might be faulty. However, the resistance of the spark plug depends on the manufacturer and can have different values.
  • Furthermore, with one probe of the multimeter at the plug terminal, you should not get any resistance reading on any other part of the spark plug apart from the central electrode. This indicates a good spark plug.


Step 2: Test The Ground Side Of The Spark Plug

  • The ground electrode is a small metal piece shaped like a "J" and ends right over the center electrode of the spark plug with a little gap.
  • Move the dial of the multimeter to the "Continuity" setting.
  • Place one probe of the multimeter on the base and the other on the ground electrode of the spark plug.
  • You should hear a beep and the reading 0 (zero) indicating a proper continuity.
  • If you do not get a reading of zero or a beep, make sure you clean up the ground electrode so that the carbon buildup is not blocking the test readings.

Using A Spark Plug Tester (Under 20 Minutes)

  • Grab a spark plug tester and attach its male end to the ignition wire and the female end to the spark plug which is attached to your engine.
  • Now start the engine and look at the bulb on the spark plug tester, which should flash with every spark. If you notice no glow or flash, then you can be sure that no spark is occurring at the electrodes of the plug.
  • This doesn't necessarily mean that your spark plug is bad. It can also happen due to a fault somewhere else in the ignition system, which needs further diagnosis.


Testing The Spark And Spark Color (15 Minutes)

  • With the spark plug removed from the engine, connect it to the ignition wire and place it on a grounded surface. A grounded surface can be the cylinder head of the engine.
  • Position the lower metal body of the plug on the cylinder head and crank the engine to notice the spark at the electrodes.
  • If you notice a strong blue spark arcing between the central electrode and the ground electrode, then your spark plug is fine. A yellow or a dull orange spark is a sign of a weak spark.
  • If no spark occurs, then it obviously means that your spark plug isn't working properly. Repeat the process with a new plug to check if the old spark plug is faulty or not.


As you can see, there are plenty of ways that you can diagnose a spark plug. Often a premature spark plug failure can showcase a bigger issue with the engine, such as a bad ignition coil, fuel injectors, etc. Just because your plug isn't firing doesn't mean it is the only culprit. Simple spark plug diagnostic tests like these help you tackle minor automotive repairs and discover potential problems with other electrical components of your vehicle. With that said, knowing how to test a spark plug pays off in the long term as it allows you to pinpoint the issue and prevent further issues.

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John Framigllia
Technical Writer
Our technical writer is known for simplifying automotive parts and services. Intuitive with various vehicles and manufacturers, he knows how to simplify complicated problems.