How To Change a Lawn Mower Spark Plug

Just like your yard needs regular care to stay healthy and thriving, your lawn mower needs maintenance to stay at peak performance. If you tend to your mower’s engine when necessary, you can keep it running longer, thereby saving you money on what would be an expensive repair or purchase. Fortunately, you can do much of this maintence yourself; this includes changing the spark plug. But what if you’re not sure how to remove the spark plug wire or even if the plug needs to be replaced?

We've got your covered. Below is a step-by-step guide to DIY spark plug replacement. It’s a straightforward, easy process that even beginners can approach with confidence.

How To Change a Spark Plug On a Lawn Mower

1. Pay Attention to Signs Your Spark Plug Needs Changing

Spark plugs aren’t meant to last forever. Over time, they wear out, which affects how the mower’s engine runs. You should replace this component either every season or every 100 hours of use to ensure your engine stays running smoothly.

However, you may have to change the spark plug in between the recommended times due to damage. How can you tell there’s something wrong? Here are common signs that you should take a look at your mower’s engine:

  • Fuel efficiency has significantly decreased
  • Difficulty getting the mower started
  • Engine suddenly stops running

2. Purchase the Correct Type of Spark Plug

Spark plugs aren’t universal; you have to make sure that you buy one that fits your mower. You can consult your owners’ manual to determine the types your lawn mower can use or take a look at the plug’s ceramic insulator, which usually has the number printed on it.

If you no longer have the manual or the number has rubbed off, you can always remove the plug first and take it to AutoZone with you. In the event your mower is missing its spark plug, write down the engine’s make and model, then consult with the associates at your local parts store. They should be able to find acceptable replacements.

3. Position Lawn Mower

Before you get to work, set your mower on a flat, stable surface. A garage floor or porch is perfect. This gives you unfettered access to the engine while ensuring your safety, as you may be injured if the mower shifts or falls while you’re performing maintenance.

You should also take this time to gather the necessary tools:

  • Screwdriver
  • Spark plug socket
  • Drive socket wrench in 1/2 inches or 3/8 inches, whichever is appropriate for your machine
  • New spark plug
  • Feeler gauge
  • Spray-on cleaner
  • Wire brush
  • Sturdy knife

Make sure the engine is cool enough to work on before you proceed. If you used the mower earlier, give it half an hour at least for the parts to cool down. Otherwise, you risk burning yourself on the metal pieces.

4. Disconnect Spark Plug Lead

If your engine has a cover, use the screwdriver to remove it. You’ll then need to disconnect the spark plug lead, also called the ignition cable. This is a wire that links the plug to the voltage source and distributor, thereby providing electricity. Make sure the connection is fully severed for your safety.

You can also take this time to examine the ignition cable. Time and use can lead to wear, which may affect your mower’s performance. Look for damaged jackets on the wires; if you see broken plastic, its time for a replacement.

5. Remove Spark Plug

Fit the spark plug socket over the plug and turn counterclockwise to loosen. If you’re having trouble ratcheting, don’t force it, as this may cause damage. Instead, apply lubricant and allow to soak for a few minutes. You should then be able to easily remove the plug.

It’s important that you specifically use a spark plug socket, as these are specifically designed to protect the ceramic insulator. Attempting to remove or install a spark plug with inappropriate tools can result in damage.

6. Clean and Inspect Plug

The plug will likely be coated in build-up, so be sure to clean it using the spray-on cleaner and your wire brush. If you encounter stubborn deposits, use the knife to scrape or chip them away.

Next, take a look at the component. If the part is in good condition and there’s still time left on its advised use period, you can put it back. However, if you notice any burned electrodes or cracked ceramic, you need a new piece.

7. Adjust Gap Using a Spark Plug Gauge

Before you can begin installation, you need to make sure the plug has the correct gap. Measure the gap between the curved and straight electrodes using the spark plug gauge. If you have the correct size for your make and model, it should only need minor adjustment. To adjust the gap, carefully bend the curved electrode with the gauge.

8. Install New Spark Plug

Put the new plug into place and manually turn clockwise. Once the threads catch, use the socket to tighten further. Be careful not to overtighten or you can break the more delicate parts. To safely install the plug, only rachet until it stops going down, then give it one last quarter turn.

9. Reconnect Lead

Now that your spark plug is back in place, reconnect the lead. If you’ve gotten a new cable, this is a good time to install it. Replace the hood of your mower and tighten the screws. Congratulations; you’ve successfully changed your spark plug.

10. Dispose of Damaged Parts

If you’ve done a full replacement, not just cleaning, you now need to dispose of the old parts. Plugs are safe to throw in the garbage, which you are welcome to do. However, if you have other auto scrap on hand or don’t mind waiting until you do, you can actually sell these parts to a scrap yard to make some extra cash.

Now that you’ve learned how to change a spark plug in a lawn mower, you can keep your equipment running smoothly without having to hire someone else for maintenance. With this skill in your repertoire, you can look forward to more cash in your wallet, an efficient mower, and a beautiful lawn.

Advice, how-to guides, and car care information featured on and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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