How to Check Lawn Mower Spark Plugs

You awaken one morning to the realization that the lawn has not been maintained in recent times. You get up, make yourself some coffee, eat some breakfast, and then get ready to cut the grass that has become ridiculously long. You quickly grab your equipment and begin to pull the lawn mower. You suddenly have the realization that the mower is not picking up the ignition at this moment. It causes you a great deal of agitation. Your train of lofty aspirations has been derailed at this point. The faulty sparkplug is the most obvious perpetrator of the crime.

But before you assign the entire responsibility for the problem to the sparkplug, hear me out. There is a possibility that the problem lies with one of the other components of the ignition system. It is important to perform a thorough inspection of all components before making any adjustments or repairs. This is where we will put the old sparkplug through its paces. Another possibility is that you have a sparkplug lying around in your backyard and you are curious as to whether or not it is any good.


How to check a lawnmower sparkplug

  • Step 1: Remove the sparkplug wire
  • Step 2: Remove the sparkplug
  • Step 3: Give the sparkplug a visual check
  • Step 4: Clean the electrodes if they are carburized
  • Step 5: Use a Digital multimeter to check continuity
  • Step 6: Check the leaks of current using DMM
  • Step 7: Check the resistance range if the sparkplug has a resistor
  • Step 8: Reinstall the sparkplug
  • Step 9: Use sparkplug tester to check the spark production

What is a sparkplug?


It is necessary to define the construction and operation of a sparkplug in order to provide a better understanding to the repairer. You will have a greater interest in learning about the condition of the sparkplug once you have an understanding of the physics and mechanism that underlies its behavior. The inspection of the sparkplug is not a particularly challenging job, but it is an essential procedure for the owner of a lawnmower to be familiar with.


Detailed construction


All of the sparkplug’s individual parts will be broken down and examined in this section.

  • The current from the ignition coil is carried by the central electrode, which is present throughout the entirety of the sparkplug’s center and is responsible for its central location.
  • The ground electrode is the side-mounted electrode that is responsible for returning the spark to its source.
  • Electrode gap: The space that exists between two electrodes and serves as the conduit for high-voltage current to jump and produce the spark.
  • Copper core: a complete range plug that functions well at both high and low speeds. Core is deeply inserted in the center electrode for better heat conductivity.
  • Present in models denoted by the letter “R” on their packaging, the resistor acts as a barrier between the high voltage current and the electrode, preventing the latter from exploding.
  • Make sure the pressure is kept inside the engine cylinder by checking the gasket.
  • Zinc plating and chromating are applied to the exterior of the metal shell to prevent corrosion.
  • Insulator: Made of high purity alumina, offering superior electrical insulation, enhanced heat dissipation, and enhanced resistance to thermal shock
  • Exceptional hermeticity and a rock-solid build characterize this special packaging.
  • The part number that is imprinted on the ceramic portion of every sparkplug may be different from one manufacturer to the next for the same type of plug.
  • Corrugations: To stop the flashover from happening. Instead of producing a spark through the gap, flashover refers to the conductance of high voltage current directly to the ground.
  • A spark plug’s terminal is the point at which its wire is connected to the ignition coil so that current can flow from the coil to the spark plug.

Working of sparkplug and ignition system


The electro-mechanical mechanism that supplies the machine with the initial power it needs to start is referred to as the ignition system.

  • A battery is a device that can turn chemical reactions into usable electricity. Holds the charge and recharges itself via the generator, which is driven by a belt that transfers power from the crankshaft.
  • The initial push that allows the engine to carry out the first cycle of intake and compression is provided by the starter motor and rope.
  • Transmission from the flywheel that is turned to move the piston in the cylinder in the desired direction according to the requirements.
  • Ignition coil: Performing the function of a transformer, it raises the voltage of the 12 V coming from the battery to an adequate level for spark generation.
  • When the spark is required inside the cylinder during the power stroke, the relay enables the current to flow at a precise timing so that the spark can occur.
  • The sparkplug is the part of an internal combustion engine that makes direct physical contact with the combustion chamber and is responsible for producing the spark that causes a mixture of fuel and air to burn and generate power.
  • The ignition system is comprised of all of these different parts, which all work together. Once the engine has started, the starter rope and flywheel are no longer required to do any work; however, the remainder of the cycle continues and ensures that the engine continues to operate.


Step by Step guide to check the sparkplug


● Step 1: Remove the sparkplug wire

After locating the plug, removal of the plug wire is performed. The wire can be pulled off using hands. It is preferred to use a sparkplug wire puller, ensuring the insulation does not get damaged.

● Step 2: Remove the sparkplug

If you want to remove the sparkplug, either a socket wrench or a sparkplug wrench tool can be used. The specialized tool has an extended neck of the socket and makes the job much more comfortable. Follow these stepwise instructions

  • Fit the wrench in place: Connect the wrench to the base of the plug
  • Loosen and unscrew the plug: Apply firm force to loosen up the sparkplug and then unscrew with ease with normal hand force
  • Removing old sparkplug: When unthreaded completely move the old sparkplug out

● Step 3: Give the sparkplug a visual check

Three things can be checked visually in a sparkplug

  • Ceramic insulation: check if there are any cracks in the white ceramic part of the sparkplug. If it is damaged, then most probably the plug is useless as it will be flashing over
  • Electrodes: check whether both of the electrodes are still present because sometimes they might blow up. In this case, it is a wise decision to throw it away.
  • Terminal: Check for any carbon deposits on the terminal side. It is sporadic to occur but easily observable.

● Step 4: Clean the electrodes if they are carburized

If there are carbon deposits on the electrode surface, it is better to clean them first to observe the best checking readings.

  • Use a soft wire brush
  • Brush through the neck thread and sides first
  • At last, give a gentle brush to the electrode
  • It is advisable not to use any abrasive methods like sanding or grinding as it will cause permanent damage to the plugs

● Step 5: Use a Digital multimeter to check continuity

  • Set the DMM in the current mode
  • Touch the probes together to check for proper functioning
  • Put the sparkplug on an insulator surface (wood or cloth) and not on a metal vise or floor
  • Touch one probe to the tip of the central electrode and the other to the terminal
  • The DMM should give a beep and offer a 0 reading
  • It shows that the central electrode is not broken 

● Step 6: Check the leaks of current using Digital Multi Meter

  • Step 1: Now instead of touching the other probe to the central electrode, connect it to the ground electrode
  • Step 2: The open-loop error should be visible
  • Step 3: Touch the probe on the gasket, insulation, and hex nut
  • Step 4: All should have an open-loop
  • Step 5: If the current conducts , the sparkplug is bad

● Step 7: Check the resistance range if the sparkplug has a resistor

If the spark has a resistor, It will have a letter R in the imprinted code. Instead of zero reading, a value of resistance will be observed. If this resistance is from 4-7 ohm, the sparkplug is okay. If higher or lower reading is followed, the sparkplug is terrible. Some plugs have a regular reading of 9 ohms, but in that case, it is generally mentioned as the last digit in code, i.e., 9.

● Step 8: Reinstall the sparkplug

  • Mind the electrode gap: Before reinstalling the sparkplug, it is imperative to gap it. Using the gap gauge or feeler gauge, choose the specific gap dimension a push between the electrodes.
  • Place the sparkplug in place: After gaping is completed, gently screw the new plug into the initial thread. Be as gentle as possible so that threads are not damaged.
  • Tighten the sparkplug: Screw with hand force and tighten using a wrench tool. Make sure not to overtighten because it might damage the plug, its seal, and threads

● Step 9: Use sparkplug tester to check the spark production

  • Installing the tester: Install one end to the terminal of the sparkplug, and the other end is connected to the connector wire
  • The check step: Pull the starter rope. A spark will be produced inside the transparent chamber. It proves the sparkplug is good

If the sparkplug is able to pass each of these tests, it will be able to function properly. In the event that the engine will not turn over, you will still need to move one and inspect the other ignition and air intake parts, such as the carburetor.


Related Question

1. What to do to make sure the spark plug does not stick when changing next time?

It is absolutely necessary to apply an anti-seize compound all the way around the thread of the sparkplug before threading it.


2. Is there any more straightforward method?


It is possible to check the engine by replacing the sparkplug with a new one after first removing the old one. If the engine starts after doing this, it is evidence that the older sparkplug was of poor quality and should be replaced.




The sparkplug is a component that must be present in order for the machine to be started. Understanding how it operates and being aware of the criteria that should be used to evaluate its performance are both topics that will be covered here. Every person who owns a lawnmower or any other type of garden equipment ought to have in-depth knowledge of sparking and the process of testing the condition of sparkplugs.

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