How To Tell If Spark Plug Is Bad Lawn Mower?

To determine if a spark plug is bad in a lawn mower, inspect it for signs of wear, such as fouling, carbon deposits, or a damaged electrode. Additionally, check the spark plug gap and ensure it’s within the manufacturer’s specifications. If the plug appears worn or if the gap is incorrect, it’s likely time for a replacement to ensure optimal engine performance.

If your lawn mower isn’t running smoothly, a potential culprit could be a bad spark plug. Identifying a faulty spark plug is crucial for maintaining your lawn equipment.

In this guide, we’ll show you simple and easy-to-understand ways to tell if a spark plug is a bad lawn mower. It ensures your lawn mower stays in top shape.

How to Tell If Spark Plug Is Bad Lawn Mower in 6 Steps

Step 1: Difficulty Starting the Mower

One of the most common signs of a bad spark plug is difficulty starting your lawn mower.

When the spark plug is in good condition, it produces a strong spark that ignites the fuel mixture in the engine.

However, if the spark plug is bad, it may struggle to generate a sufficient spark, making it harder to start the engine.

If you find yourself repeatedly pulling the starter cord without success. It’s a good indication that your spark plug might be the culprit.

Step 2: Rough Engine Idling

A healthy lawn mower engine should run smoothly at a consistent speed when idling. If you notice that your mower’s engine is idling roughly.

Or experiencing irregular fluctuations in RPM (revolutions per minute), a bad spark plug could be the cause.

The uneven spark from a faulty spark plug can disrupt the combustion process, leading to engine instability.

Step 3: Reduced Engine Power

A decrease in engine power is another sign that your spark plug may be bad. When the spark plug fails to ignite the air-fuel mixture properly, the engine loses power, resulting in sluggish performance.

You may notice that your mower struggles to cut grass effectively. It takes longer to complete the job than it used to when the spark plug was in good condition.

Step 4: Increased Fuel Consumption

A bad spark plug can also lead to increased fuel consumption. When the spark plug doesn’t fire efficiently, the engine may not burn the fuel as completely as it should.

This incomplete combustion wastes fuel and can cause your mower to run less efficiently, resulting in more frequent refueling.

Step 5: Blackened or Fouled Spark Plug

To visually inspect your spark plug, you can remove it using a spark plug socket and examine its condition.

A healthy spark plug should have a light tan or grayish appearance on the electrode and insulator.

However, a bad spark plug may appear blackened, fouled, or covered in carbon deposits. This is a clear indication that it needs replacement.

Step 6: Weak or No Spark

In some cases, you can directly check for a spark by removing the spark plug from the engine and reattaching it to the spark plug wire.

Ground the spark plug against the engine block while pulling the starter cord. You should see a visible spark at the tip of the spark plug.

If there’s little to no spark or it appears weak, it’s a strong sign that the spark plug is bad and needs to be replaced.

Some Common Symptoms of a Bad Spark Plug

  1. Rough Engine Idle: When a spark plug is bad, your engine may start to idle roughly, causing noticeable vibrations.
  2. Reduced Fuel Efficiency: A faulty spark plug can lead to decreased gas mileage, meaning you’ll have to fill up your tank more often.
  3. Difficulty Starting the Engine: If it’s harder to start your car, a bad spark plug could be the culprit, making ignition a struggle.
  4. Misfiring Engine: A bad spark plug can cause your engine to misfire, leading to a noticeable loss of power and performance.
  5. Poor Acceleration: Sluggish acceleration can be a sign of a bad spark plug, as it hinders the engine’s ability to produce power quickly.
  6. Increased Emissions: Faulty spark plugs can contribute to higher emissions, potentially causing your vehicle to fail emissions tests.
  7. Engine Hesitation: You may experience hesitation or a delay in response when you press the gas pedal due to a bad spark plug.
  8. Rough Engine Sound: A poorly functioning spark plug can lead to a rough, uneven engine sound, like it’s not firing smoothly.
  9. Stalling: In extreme cases, a bad spark plug can cause your engine to stall, leaving you stranded or in an unsafe situation.
  10. Check Engine Light: If your vehicle’s onboard computer detects spark plug issues, it may illuminate the check engine light on your dashboard.
  11. Excessive Fuel Odor: A bad spark plug can result in incomplete combustion, leading to unburned fuel and a noticeable gasoline odor.
  12. Engine Overheating: In some cases, a bad spark plug can contribute to engine overheating, as it can’t efficiently burn fuel, causing excess heat.

Will a Bad Spark Plug Cause a Mower Not to Start?

A bad spark plug can indeed prevent a lawn mower from starting. Spark plugs play a crucial role in the ignition process of a small engine like a mower.

When the spark plug is in poor condition, it can’t produce a strong spark, which is necessary to ignite the fuel-air mixture in the engine. This leads to difficulty starting the mower, poor performance, and even engine misfires.

Regularly inspecting and replacing spark plugs as part of your mower’s maintenance routine is essential for ensuring reliable starts and optimal engine performance.

If your mower is having trouble starting, checking and replacing the spark plug is a good place to begin troubleshooting.


The spark plug in your lawn mower plays a crucial role in its proper functioning. Recognizing the signs of a bad spark plug, such as difficulty starting, rough idling, and reduced power.

Its increased fuel consumption, or a visibly fouled plug, is essential for maintaining your mower’s performance.

Regularly inspecting and replacing spark plugs as needed will ensure that your lawn mower runs smoothly and efficiently. It allows you to keep your lawn well-maintained throughout the mowing season.


What are the symptoms of a bad spark plug in a lawn mower?

The symptoms of a bad spark plug in a lawn mower include difficulty starting the engine, rough idling, reduced power, increased fuel consumption, and a tendency to stall. Additionally, you may notice black, sooty deposits on the spark plug or experience frequent misfires. Regular spark plug maintenance is crucial for optimal lawn mower performance.

Will a lawn mower start if the spark plug is bad?

A lawn mower will not start if the spark plug is bad. The spark plug plays a crucial role in igniting the fuel-air mixture in the engine. If it’s faulty, there won’t be a spark to ignite the mixture, preventing the engine from starting. Replacing the spark plug is a common solution to this issue.

Can you clean a lawn mower spark plug?

You can clean a lawn mower spark plug. Start by removing the plug and using a wire brush or a spark plug cleaner tool to remove carbon deposits and dirt. Ensure the electrode gap is correct, typically using a gap gauge. Reinstall the spark plug, and your lawn mower should run more efficiently.

What causes a weak spark on a lawn mower?

A weak spark on a lawnmower can be caused by several factors. Common culprits include a fouled spark plug, damaged ignition coil, faulty ignition switch, or a loose or damaged spark plug wire. Addressing these issues through proper maintenance and replacement can typically restore a strong spark and improve the mower’s performance.

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