How to Test a Spark Plug on a Lawn Mower (Step By Step)

Knowing how to test a spark plug on a lawn mower is something that can really come in handy. The best part is, once you know how to test one spark plug, you know how to test them all. Whether you’re dealing with a snowblower, string trimmer, chainsaw, or even a motorcycle, you’ll have a surefire method to follow. There are plenty of situations where you could find yourself wanting to figure out if a spark plug still works, and luckily it isn’t too hard to get answers.

You Might Want to Test Your Lawn Mower Spark Plug If…

The main signs that potentially indicate a bad lawn mower spark plug are:

• Trouble or failure starting the mower.
• Loss of power.
• Running out of gas more quickly.
• Oil leaking from the exhaust, oil reservoir, or crankcase.

Equipment You’ll Need to Test Your Lawn Mower Spark Plug

In order to give your spark plug a quick examination, you will need a couple of tools. Most of these tools are pretty common, and if you don’t have them already, they’re worth the investment.

Here’s what you’ll need:

• Socket wrench
• Spark plug socket
• Insulated pliers
• Paper towel or clean rag

Optional Equipment:

• Multimeter
• An extra set of hands (if not using a multimeter)

Quick Overview of Testing a Lawn Mower Spark Plug

Let me go through a very broad description of how to test a spark plug on a lawn mower before I dive into detail.

• The very first thing you’ll need to do is get your spark plug removed from the mower.
• With the spark plug removed, the next step is to clean/wipe the spark plug off.
• Then, conduct your test. You’ll either do this with a visual test or a multimeter.
• Lastly, take the test into account and decide whether to replace or reinstall. Either get your hands on a new spark plug, or if your current one works, you can throw it back in the mower.

How to Test a Spark Plug on a Lawn Mower (Step By Step)

Now that I’ve outlined the basic steps for how to test a spark plug on a lawn mower, let’s talk details. I’ll go into depth about testing spark plugs both with and without a multimeter.

1. Remove Spark Plug

The process of removing a lawn mower spark plug is pretty straightforward with the right tools. Pop the spark plug hood off, secure the ignition cable away from the plug, and get your socket set on it. One thing to keep in mind here is that you’ll want to be sure you have a good grip on the spark plug and gently twist it loose so that it doesn’t strip.

2. Test Spark Plug Visually (Without Multimeter)

To conduct a test for spark without a multimeter, I would recommend asking a friend or neighbor to lend a hand. The following steps will give you a good idea of why.

First, reconnect the ignition cable to the spark plug once you have the plug removed. Then, grasp the spark plug by the threads or porcelain body with your insulated pliers and ground the spark plug. To do this, you just need to press the end (near the threads and electrode) against a piece of metal such as the engine block or mower deck.

With the spark plug grounded, the next thing to do is to turn the engine over. This is where your extra set of hands comes in. One person needs to pull the starter rope or turn the key if you have an electronic ignition. Since the spark plug is out of the engine, you don’t need to worry about the engine starting.

Now, you or the person holding the spark plug needs to focus on the electrode and look for a blue spark arcing when the motor turns. A healthy spark plug will produce a vibrant blue spark each time the motor turns.

Be extremely careful when doing this. Make sure the insulation on your pliers is in good shape, and don’t touch the spark plug with your hand. Though the amperage is low and not fatal, you don’t want to learn what a few thousand volts feels like.

3. Or, Test Spark Plug With Multimeter

Alternatively, if you have the tool, figuring out how to test a spark plug with a multimeter isn’t super complicated. Here I’ll describe the process for doing a resistance test in ohms.

I think the trickiest part is being sure to use the right setting. Set your multimeter dial to either 10k, 20k, or higher, in ohms. Most multimeters will say OHM on them or simply represent it with the omega (horseshoe) symbol: Ω

Now take one of the multimeter leads and set it against the metal tip that the spark plug hood snaps onto (the terminal). I like to set the spark plug on a table on its side and press the lead on the side of the terminal so that it doesn’t roll around.

Then, take your other multimeter lead and put it on the very tip of the spark plug’s electrode. This electrode is on the side of the spark plug that screws into your engine. The electrode is the metal part that barely sticks out of the porcelain sheath, not the curved piece of metal connected to the threads. You want your multimeter lead to be resting at the flat tip (or top) of the electrode, not on the cylindrical side. It’s important to do this the right way so that you have an accurate reading.

Double-check that your multimeter is turned on and is at the right setting, and then take a look at the reading. Ohms are displayed in the ones or tens on a multimeter, but represent thousands. A functioning lawn mower or small engine spark plug will read between five and fifteen thousand ohms on a multimeter resistance test.

An example of what this will actually look like on the screen of your multimeter is: 5.00 (five thousand ohms) or maybe 15.00 (fifteen thousand ohms). If you get a reading in this range, your spark plug works. If not, it’s no good.

4. Replace or Reinstall Spark Plug

Now you’ve got to decide whether your spark plug is good enough to keep using or whether it should be replaced. The general signs that a spark plug is still functional are either, you see a good spark when doing a visual test, or you get a reading between 5,000-15,000 ohms on your multimeter. If this is the case, you can keep using your current spark plug.

If you’re not noticing a strong spark, there is no spark, or you aren’t getting a good multimeter reading, it’s time to replace the spark plug.

If in Doubt After Testing the Spark Plug…

What isn’t much fun is doing all the necessary testing and still being unsure what to do after carefully examining your spark plug. But as it happens, I’ve got a very clear guideline to follow: when in doubt, go and buy a new spark plug. They are inexpensive, and when it comes to how often to replace a spark plug, they wear out after about one season’s worth of mowing anyways.

If you’ve gone through the trouble of pulling the spark plug out already, you might as well get ahead of your annual maintenance. Actually, one of the only situations where I’ll reuse my current spark plug is when I’ve removed it and tested it to help chase down a different mechanical problem.