There may come a time when you are not getting a spark from your spark plug, and you want to know why. Several different things on your mower can cause a lack of spark, so knowing how to test a spark plug to rule out potential problems can be really handy when diagnosing an electrical system. Let me share with you an easy-to-follow method on how to test a lawn mower spark plug.
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How to Test a Spark Plug on A Lawn Mower
To test a lawn mower spark plug, you’ll need to use a multimeter and carry out the following tests depending on your specific spark plug. Lawn mowers can use two types of spark plugs: resister and non-resister.
- Resistor Spark Plugs: Test the resistance of the center electrode and the continuity between the center electrode and the side electrode.
- Non-Resistor Spark Plugs: Test the continuity of the center electrode and the continuity between the center electrode and the side electrode
Why it’s Important to Test the Spark Plug on Your Lawn Mower
With every revolution of your mower’s flywheel, the coil sends a charge to the spark plug. This is because the spark plug is grounding the charge and allowing the charge to escape. So, if you have a faulty spark plug, the charge has nowhere to go and will damage your lawn mowers coil. As a result, the more times the charge can’t be released from the coil, the more damage is caused until the coil burns out. If this happens, you’ll need to replace the spark plug and the coil.
If you need help finding the spark plug, check out my article on where is the spark plug on a lawn mower.
How Often Should You Test the Spark Plug on Your Mower
If your lawn mower is running smoothly, then you’ll probably only need to replace your lawn mower spark plug every couple of years. Testing the spark plug once a year when getting your lawn mower ready for spring should be enough. But if you have a few performance issues, mechanical issues, or even bad fuel, you might have to test your spark plug more frequently. Here are five common symptoms of a bad spark plug to look out for.
Symptoms of a Bad Spark Plug
- Trouble Starting Your Mower
- Lawn Mower Starts then Dies
- Engine Misfiring
- Engine Surging
- High Fuel Consumption
- Lack of Power
What You’ll Need to Test Lawn Mower Spark Plug
- Spark Plug Wrench
- Non-Conductive Surface/Cloth
- Multimeter (Resistance/Continuity)
- Wire Brush
Testing a Spark Plug On a Lawn Mower With a Multimeter
To test your spark plug, you’ll need to use an electrical multimeter that can test for resistance and continuity. These are fairly inexpensive tools that you should be able to purchase from your local hardware store if you don’t already have one. Next, you’ll need to perform a few preparation steps before testing your spark plug. Here’s what you’ll need to do.
Remove the Spark Plug from the Lawn Mower
First, use a spark plug wrench to remove the spark plug from your lawn mower.
Clean the Spark Plug
Next, use a small wire brush to clean the top of the spark plug, which is normally screwed into your lawn mower’s engine. You need to make sure that you remove any carbon build-up that could be connecting the center and side electrodes.
Place the Spark Plug in a Nonconductive Surface
Finally, place your spark plug on a non-conductive surface, such as a wooden workbench or on top of a cloth. This is so that the electrodes are not connected through your work surface.
How to Test a Lawn Mower Resistor Spark Plug
Here are the two tests you’ll need to complete to test a resistor spark plug.
Center Electrode Test
- Set the multimeter to resistance.
- Connect one probe of the multimeter to the terminal of the spark plug.
- Connect the second probe to the center electrode.
- Review the multimeter results.
Center Electrode to Side Electrode Test
- Set the multimeter to continuity.
- Connect one probe of the multimeter to the terminal of the spark plug.
- Connect the second probe to the side electrode.
- Review the multimeter results.
How to Test a Lawn Mower Non-Resistor Spark Plug
Here are the two tests you’ll need to complete to test a non-resistor spark plug.
Center Electrode Test
- Set the multimeter to continuity.
- Connect one probe of the multimeter to the terminal of the spark plug.
- Connect the second probe to the center electrode.
- Review the multimeter results.
Center Electrode to Side Electrode Test
- Set the multimeter to continuity.
- Connect one probe of the multimeter to the terminal of the spark plug.
- Connect the second probe to the side electrode.
- Review the multimeter results.
Lawn Mower Spark Plug Test Results
Now that you have tested your lawn mowers spark plug, let’s go over what the results should be and what actions to take if you got different results.
Resistor Spark Plug Test Results
- Center Electrode Test = At or slightly below the spark plug specification for resistance.
Example NKG-BPR5ES 5000 Ω Resistance
If the results are above the spark plug resistance specification, then there is too much resistance, and the spark plug should be replaced.
- Center Electrode to Side Electrode Test = No Continuity
If continuity is present between the center and side electrode, then the spark plug should be replaced.
Non-Resistor Spark Plug Test Results
- Center Electrode Test = Yes Continuity
If there is no continuity between the spark plug terminal and the center electrode, the spark plug should be replaced.
- Center Electrode to Side Electrode Test = No Continuity
If continuity is present between the center and side electrode, the spark plug should be replaced.
