5 Lawn Mower Bad Spark Plug Symptoms to Look Out For!

Lawnmowers with gasoline engines require enough ignition to work, and spark plugs are the primary source that fires the energy to run the engines. However, a bad spark plug makes it difficult to run or even start your mower, and if not repaired on time, it can cause major harm to your engine.

Fortunately, you can avoid such a ruckus by replacing the spark plug as soon as you observe any of the following symptoms:

  • Difficulty starting the mower engine,
  • Insufficient engine performance or failure, and
  • Increased fuel consumption.

This article will explain the lawn mower bad spark plug symptoms and how to get rid of a bad spark plug.

Lawn Mower Bad Spark Plug Symptoms

5 Lawn Mower Bad Spark Plug Symptoms to Look Out For:

Learn more about each symptom that warns about a bad spark plug on your mower:

1. No Start or Starting Difficulty:

No starting or hard starting is the most common and evident sign of a lawn mower’s bad spark plug.

The engine of a lawn mower relies on the spark from the spark plug to ignite the aerosolized fuel that enters the combustion chamber. If the spark plug is faulty, it will not produce the necessary spark for proper combustion.

As a result, the combustion will not propel the piston within the engine to fire up, resulting in the no-start or hard start problem.

Hard starting is usual as the mower ages. But what if your mower requires many more pulls of the starter pull cord to fire up the engine or won’t start even after you turn the key several times?

This signifies that the spark produced by the spark plug is insufficient to ignite the fuel and air mixture in the cylinder.

Every time you try to start the engine, more fuel is poured into the cylinder. The more fuel there is, the more difficult it will be to start the engine.

If you continue to start the mower in this situation, the engine will flood, inflicting more damage to the mower’s key components.

2. Insufficient Engine Performance or Failure:

Even if your mower engine starts after several attempts, it will not run for an extended period due to a faulty spark plug and will either die out or cease mowing.

This happens due to the increased engine temperature, which widens the electrode gap and weakens the strength of the generated spark output.

A defective spark plug frequently generates a misfire or strained sound in an engine cylinder. When mowing, you may notice that the engine struggles to restart or entirely stalls.

A fouled or weak spark plug might cause your mower to not only run rough but it may also cause the engine to sputter, pop, or backfire.

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3. No, or Lack of Power & Rough Idling:

If your mower fails to cut the grass that it normally cuts with ease, the engine may not be getting the spark it needs to operate at peak performance. That’s another clear sign that something is wrong with the spark plug.

As the spark plug wears out, it stops producing enough sparks, and the engine will not be able to work effectively owing to insufficient combustion.

A weak spark plug may lead you to experience low or no engine power. You may also notice rough idling, odd vibration or tremor while idling, and a drop in engine RPMs.

4. Increased Fuel Consumption & Less Fuel Efficiency:

Another red flag that customers should be aware of is when their mower suddenly starts using more fuel for the same work than usual, and they must refuel it more frequently than usual.

You will typically notice a significant drop in fuel efficiency when the spark plug has an issue.

Since a bad spark plug can’t provide enough ignition and the fuel within your engine isn’t properly combusted, you’ll use more fuel and get less mileage.

You may even notice fuel burning odor coming from the engine or fuel assembly of your mower due to a bad spark plug.

5. Physical Flaws in Spark Plug Appearance:

Some people may miss the notice at each scheduled maintenance session due to a lack of knowledge.

 Spark Plug
Spark Plug Appearance

During routine maintenance, keep an eye on the spark plug on your mower for the following physical signs:

  1. Cracks or chips in the white porcelain insulator of the plug
  2. Electrodes with rounded or pitted edges
  3. The electrode end is black due to the built-up carbon
  4. The spark plug has turned black
  5. The space between the electrodes is too large.

Related Post: 5 Reasons Why My Mower Won’t Turn Over Unless Spark Plug Is Out?

What Should You Do If Your Lawn Mower Shows Signs of a Bad Spark Plug? (Step by Step Guide)

Follow the below steps as soon as you notice or encounter any of the earlier discussed bad spark plug symptoms on your mower:


The signs of a bad spark plug are obvious but it is important to investigate further to confirm if the spark plug is faulty since other key components may also show similar symptoms when they turn bad.

So, start the task by removing the spark plug wire and moving it out of the way to get better access to the spark plug.

Take a 5/8” x 3/8” spark plug socket or wrench and place that over the spark plug. Until you can feel that it has engaged on the Hex install the turning tool. Loosen and take the spark plug out slowly with a few turns.


Once you have successfully removed the spark plug, it’s time to properly inspect the current spark plug and understand the actual problem.

Now see if the spark plug looks like it’s covered with carbon and dark. If it is, then use a soft wire brush to clean the spark plug and start brushing it carefully from the outer edge of your spark plug.

Make sure not to damage the porcelain insulator around the electrode.

You can even use a spark plug cleaner spray to clean the stubborn dirt if necessary.


Once the spark looks clean, you should check the gap between the spark plug electrode.

Use a spark plug gapping tool for this task. Insert that gapping tool between the open and gap in the spark plug on the smallest end.

Then slide the spark plug up until wherever it stops and compare the measurement to whether it’s the right gap required for your spark plug or need to adjust it.


See if the spark plug is in good shape now or if it is completely worn out or damaged to be restored.

If the spark plug is completely damaged, replace it. Before purchasing a new spark plug for your mower, be sure it fits correctly with the engine.

Once you are sure that you have chosen the right spark plug for your lawn mower, install it accordingly.

Spark Plug
Spark Plug

Frequently Asked Questions [FAQs]

Are Spark plugs essential to run lawn mower engines?

Yes, they are. Because a spark plug is a small electrical device attached to the cylinder head of an engine and they are the main source to ignite the fuel and air mixture within the mower engine to achieve combustion.

Will my lawn mower run if I have a faulty spark plug?  

In most cases, when the engine spark plug gradually declines, your mower will not start or you will encounter several starting difficulties.

Why won’t my lawn mower start after I changed the spark plug?

If your lawn mower won’t start after changing the spark plug, you should check the fuel and ensure that you have enough right fuel in the tank. You should also check whether the carburetor is dirty or clogged and service the carburetor as needed.

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