How to test a spark plug on a lawn mower can be a useful skill to acquire for anyone. First, it is a straightforward procedure that barely takes an hour to accomplish. Second, if you learn how to test spark plugs on lawnmowers, you can also test plugs on all vehicles.

Read our complete guide below to learn the steps and the various methods of testing these plugs on a lawn mower.

What Are the Steps To Test a Spark Plug on a Lawn Mower?

The steps to test a spark plug on a lawn mower starts with preparing the equipment and turning off the mower. Once your lawn mower is off, take the spark plug out after cleaning it, and then use either the visual method or the millimeter method to test it.

You can also try out the tester method, which doesn’t require the removal of the plug.

1. Prepare the Equipment You Will Need

If you own a lawn mower or any other vehicle, you will inevitably have to test its spark plug. That is why investing in spark plug testing equipment is a worthwhile investment . If you want to avoid purchasing them, they can also be borrowed from a nearby hardware store.

  • A standard spark plug wrench is a must-have tool because these cannot be unscrewed using regular wrenches.
  • It helps to have a spark plug wire puller because pulling by hand might require a bit of force.
  • You will need ordinary wrenches for removing the external covering of the plug and other related equipment.
  • You need insulated pliers if you test the plug without using a millimeter.
  • A multimeter is optional but recommended for proper testing of mower plugs.
  • You will also need paper towels, rags, and grease cleaners for cleaning the equipment.

2. Get the Plug Removed First

Removing the plug from a mower involves more than just taking it out. You must follow a standard procedure for your safety and the machine’s health.

  • After turning the mower off, disconnecting its battery is the next step. Loosen the nut that connects the ground wire to the battery’s negative terminal using an appropriately-sized hand wrench. After this, you can leave the positive wire without removing it.
  • Next, expose the plug, often located behind the air filter, and disconnect its wire. Removing the spark plug wire might require a little bit of controlled force and will come off with a pop.
  • Cleaning the area around the plug is important before taking it out. Otherwise, all the dirt, grit, grime, and oil accumulated around it will fall through the hole created by the plug’s absence into the mower’s engine system. This debris might damage the mower engine, and you might find that the mower won’t start the next time you try.
  • Finally, use a spark plug wrench to unscrew it from within the mower. Any regular wrench cannot remove this. These plugs come only in one size, so that a standard plug wrench will remove all kinds of spark plugs.
  • Rotate the plug with its socket by rotating the wrench in a counter-clockwise motion. If it is still hot, put it aside until it cools down for inspection and testing.

3. Look for Signs of Physical Damage

First of all, test the plug to make sure that it has not incurred physical damage. A normal plug should appear light grey or tannish when held to a light source. It should not have any product build-up occurring on its surface.

Look for Signs of Physical Damage

Another important thing to look for here is the spark plug gap, which is the distance between its side and center electrodes. A plug gap determines the voltage needed to produce the spark that will ignite the engine.

Standard mower spark plugs should ideally have plug gaps between 0.02 to 0.035 inches to produce the perfect spark in the ignition coil. You will have to purchase a plug gauge for measuring this gap and if the gap is insufficient, adjust it accordingly.

4. Check for Signs of Carbon Fouling

When a fuel with a skewed air-fuel ratio is used consistently to power a lawn mower, you risk developing carbon fouling on its spark plug. Using a relatively higher fuel ratio can lead to carbon deposits solidifying all over the surface of the spark plug. This cannot be left as it is because it eventually starts impairing the plug’s hardware.

You do not need to worry much because carbon fouling can be easily removed using a brake cleaner and a rag. Have a serious conversation with your gas pump owner about the quality of the fuel being supplied to you.

5. See if the Plug Is Wet From Oil

Whenever oil leaks occur from a mower tank, the oil is bound to trickle down the engine all over the spark plug. You can wipe these off using rags and oil-removing cleaners that can easily be ordered online. You must complete this step so that this oil will leak into the engine once you take the plug out.

See if the Plug Is Wet From Oil

Within the engine’s parts, it can block channels and attract grime to solidify. This oil clogging would severely damage the engine in the long run. As for the oil leakage, you will have to see where it is happening and how to fix it.

6. Look for Signs of Burns or Wear

It is important to rule out signs of burns or wear on a plug because, in that case, it will have to be replaced by a new one. Blisters will appear on the insulator tips when this equipment has been burned due to overheating issues.

Other hard-to-miss signs of a burnt plug are molten plastic and burnt metal. A burnt insulator also means that your mower is overheating and its insulation needs to be fixed.

When a plug has been used for a long time, you need to say goodbye to it eventually. If the plug has been used for years, it will fall apart while removing it from the mower. It will also appear visibly worn, broken, and ready to be removed.

7. Test the Plug Without a Multimeter First

If you want to test a plug’s functioning without using a multimeter, it is better to call in a friend for help. While the plug is still removed from the mower, reconnect it to the ignition wire of the engine. Next, ground the spark plug by connecting its threads or porcelain coverings with insulated hand pliers.

Now ask your friend to turn the engine over carefully and very slowly. You will not be able to achieve it yourself if your mower is ignited by pulling a starter robe or turning the key. Look out for bright blue sparks produced in the plug after turning the engine.

There are better methods to test a plug than this, so you must be very careful when doing it. The insulation on your grounding pliers needs to be in perfect condition, and you must not touch the plug with bare hands.

8. Test the Spark Plug Using a Multimeter

A lot of people need help with using a multimeter. After reading the step-by-step guide below, you will see how easy reading a multimeter is.

Test the Spark Plug Using a Multimeter

  • Make sure to choose the right setting on your multimeter before starting and set it to ohms, which is the unit for measuring resistance. Some multimeters represent ohm by its greek symbol Ω while others will have OHM written in the settings. You can set the multimeter’s dial to 10, 20, or higher ohms per your wishes.
  • Check to see no resistance between the two probes of the multimeter. When you touch the probes together and the resistance drops to zero, you know that your meter will give an accurate reading.
  • Look for a metal tip emerging from the spark plug attached to the terminal. Take one lead of the multimeter and set it against this metal tip.
  • The tip of the second multimeter lead needs to touch the spark plug’s electrode. This electrode is the part of the plug that sticks slightly out of its porcelain sheath on the side facing the engine. To get an accurate reading, the probe needs to rest on the flat tip of the electrode instead of its cylindrical part.
  • Turn the multimeter on to see what reading your spark plug gets. The result will be represented in ohms in the ones or tens but represent the thousands. A healthy and functioning lawn mower spark plug should read between 5,000 to 15,000 ohms.
  • For example, if your multimeter gives a reading of 6.0 ohms, then this means that the actual reading is 6,000 ohms, and your mower is in perfect functioning condition.

9. Test the Spark Plug With a Tester

You can also test a potentially faulty spark plug without removing it completely from the lawn mower. Buy a spark plug tester and follow the steps given below to carry out this testing.

  • Start by turning the lawn mower ignition off and taking out its key. Then gain access to the spark plug by removing its protective covering on the side of the mower.
  • One end of the plug tester is to be attached to the ignition wire of the mower. The other end must be attached to the spark plug without disconnecting it from the engine.
  • Try putting the key back in and starting the ignition, then turn the engine on again and look at the tester’s transparent sides. A noticeable glow or spark should appear on the sides if the spark plug is working.

10. Be Aware of Safety Precautions

No matter how often you have done this, never compromise your safety while handling electric equipment.

Be Aware of Safety Precautions

  • Start by putting on thick insulating rubber gloves and eye goggles.
  • Always turn the engine off and disconnect the spark plug first because you do not want to give yourself a bad electric shock, trust us.
  • Make sure that you wait for the engine to cool down fully after it has been turned off. This might take longer in summer, but it is always worth the wait. Accidentally touching a hot plug or engine component is not good for its health and might cause burns if you are not wearing gloves.
  • Never make the mistake of directly touching a spark plug while the mower’s engine is still running. With an electric current of around 20,000 volts running through it, this can lead to lethal consequences.
  • Never put your hand near any equipment that is still moving in any capacity.


Before you go, it is important to recap this step-by-step guide on how to test a lawnmower spark plug.

  • Don insulating gloves and turn the small engine of the mower off before starting the testing in order to prevent getting electrocuted.
  • Remove the plug from its assembly within the mower, which can be done using only a single standard plug wrench.
  • Carefully inspect your plug to see if it is physically damaged or has carbon fouling because such a plug will have to be removed.
  • You can test the plug using a multimeter, without a multimeter, or through a plug tester.
  • The range of voltages exhibited by a multimeter should be between 5,000 to 15,000 ohms.

Testing the engine spark plug should be your number one priority when a lawn mower problem arises. In only a few simple steps discussed above, you can determine whether or not the problem lies in the spark plug and if there is any need to replace it.

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