How to Use a Spark Plug Tester in Quick and Easy Steps

a Spark Plug Tester

A spark plug is an essential part of the internal combustion engine (ICE), that is, your car’s engine. The spark plug is just a simple device that is screwed in your car engine’s cylinder head that delivers a spark (or an electrical signal) igniting the mixture of fuel and air inside the engine’s combustion chamber.


SPARK PLUG (Via: orangeauto)

Although this device is quite small in size, the spark plug delivers an ultimate punch to run your engine. A spark plug looses its magic (misfire or spark miss) when there is a drop in voltage (usually 12,000 to 25,000 volts or more of electricity) during ignition. The current that flows through the spark plug comes from your engine’s ignition coil or magneto.

Once your spark plug fails to “fire”, there could be something wrong in the system or the spark plug itself. Here, we will teach you how to use a spark plug tester in quick and easy steps. To diagnose what is really wrong with your spark plug, this tester will help you to find out if your spark plug needs to be replaced or not.

How a spark plug works

Basically, a four-cylinder car has 4 spark plugs, a one with six cylinders will have 6 spark plugs, and so on and so forth. However, a HEMI engine (an engine with a hemispherical cylinder head or “hemi-head”) has 2 spark plugs at each cylinder.

The spark plug is mounted at the top of the cylinder head. Initially, the piston on the cylinder head goes down the cylinder that draws a mixture of air and fuel. Then, the piston travels back toward the spark plug and then compresses the air and fuel mixture.

When the piston is at its highest or fullest reach (called the top dead center or TDC), current flows through the gap between the central and lateral electrodes of the spark plug ignites the mixture. At this point, the piston is brought back down creating power for the car, then thrust again upward clearing the exhaust. This cycle is repeated again and again which makes the engine run.

Here is the quick rundown of how a spark plug gives power to your engine (the engine’s different stroke in this order) for a four-stroke engine cycle:

  • Intake stroke
  • Compression stroke
  • Ignition stroke
  • Exhaust stroke

Types of spark plugs

There are two types of spark plugs that are available in the market: the hot and cold types.

  • Cold type

The cold type spark plugs are those that work best in high horsepower (hp) and high-compression engines. These type of plugs have less insulation that is why they can transfer heat efficiently from the combustion chamber to the exterior of the engine.

  • Hot type

The hot type spark plug is commonly used in the current breeds of cars. It has more insulation that keeps the temperature high, thus enabling to burn more carbon deposits. This makes the plug last longer than its cold counterpart.

What is a spark plug tester?

You may wonder what is a spark plug tester and do you really need this device? Read on, and find out for yourself.

A spark plug tester is a light and handy device that could tell if your spark plug is working properly or not. This tool is designed for testing of spark plugs and also the ignition system. Additionally, this tool has no batteries, no lubrication needed, and basically a stand-alone device.

A spark plug tester is also dirt cheap, with heavy duty brands costing less than $20 while some brands go as low as $10 without accessories. This device primarily saves you time and money by letting you inspect if the problem is in the ignition. The tester can also tell you the quality of the spark compared to what is normally the plug should produce.

Some car owners who don’t own a spark tester usually throws away their old spark plug when it suddenly stops sparking without the benefit of testing it. This should not be the case. We believe that a spark plug is damn cheap, but if you have too many cylinders in your engine, throwing away plugs without testing could cost you a fortune over time.

Now, it is up to you to decide if you would need a spark tester in your set of tools.

Things you will need to test your spark plug

  • Spark plug
  • Spark plug tester
  • Flashlight
  • Safety gloves

Here’s what you would do

Before you proceed with the testing of your spark plug, be sure to wear safety gloves on both hands and flashlight to light you up.

  • Checking the spark plug

  1. Disengage the plug’s high tension lead from the spark plug.
  2. Mount the tester’s prong to the high tension lead of the spark plug.
  3. Connect the tester to the spark plug. This will allow a link between the high tension lead to the spark plug.
  4. Turn on the engine and see if there is electricity (spark) flowing through the spark gap of the tester or an inconsistent timing.
  5. If there is no ignition on the engine, disconnect the tester and take out the spark plug from the engine.
  6. Clean the spark plug’s electrodes and adjust the gap between them (the gap is usually between 0.028-0.06 inch {0.71-1.52mm}). Check out your engine’s manual for the appropriate gap and how to correct the gap.
  7. After you have cleaned and adjusted the gap on your spark plug, repeat steps 1 to 4.
  8. Upon starting the engine, you will see a spark or blink on the tester’s spark gap and the engine should run if your spark plug is still in good condition. However, if the engine does not run and the spark tester did not blink, your spark plug is already dead. You can now replace your busted spark plug.
  • Checking the ignition

Checking the ignition of the engine, you must use a spark tester with an alligator clip on the other end. If not, there is a spark tester kit that provides this convenience. Like the Thexton THE404 Spark Tester available from Amazon:

This is what you should do to verify if your ignition is working

  1. Disengage the high tension lead from the spark plug.
  2. Connect the tester to the plug’s lead.
  3. Put the alligator clip on ground.
  4. By cranking the engine using the electric starter or the rewind, check out if the tester sparks.
  5. When there is spark on the gap, your ignition system is okay. But if there is no spark, you have a problem with the system. You can bring your car to an expert mechanic for thorough check up.

Some unsolicited advice

If you want long lasting spark plug, try the Bosch (4418) FGR8DQP Platinum Spark Plug. This powerful spark plug does not require gapping, so you don’t need to adjust. This heavy-duty spark plug is also available on Amazon, check this one out!

John Doe


Did you enjoy this tutorial? Although spark plugs are cheap, using them in large number could also cost you a fortune. However, if you can still salvage a reliable one or a good number of them, why not? Learning how to use a spark plug tester will certainly save you time and money! By the way, a spark tester is also very cheap.

Please tell us what you think about this article and share it if you like it.

About the author

Carolina - Care My Cars

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