How to tell if a lawn mower spark plug is bad?

How to tell if a lawn mower spark plug is bad?


A lawn mower spark plug plays a crucial role in the ignition system, providing the necessary spark to start the engine. Over time, spark plugs can wear out or become faulty, leading to poor engine performance or even failure to start. In this article, we will explore the signs that indicate a bad spark plug in a lawn mower and how to address the issue effectively.

Signs of a Bad Spark Plug

1. Difficulty Starting the Engine: One of the most common signs of a bad spark plug is difficulty starting the lawn mower engine. If you find that your mower is taking longer to start or not starting at all, it could be due to a faulty spark plug.

2. Engine Misfires: A misfiring engine is another indication of a bad spark plug. You may notice that the engine is running unevenly, with intermittent jerking or hesitation. This can result in reduced power and poor performance.

3. Rough Idle: A lawn mower with a bad spark plug may experience a rough or unstable idle. The engine may vibrate excessively or produce unusual noises while idling. This can be a sign of a spark plug that is not firing correctly.

4. Decreased Fuel Efficiency: A faulty spark plug can lead to decreased fuel efficiency. If you notice that your lawn mower is consuming more fuel than usual or running out of gas quickly, it may be a result of a bad spark plug.

How to Check a Spark Plug

1. Disconnect the Spark Plug Wire: Before inspecting the spark plug, make sure the lawn mower is turned off and the spark plug wire is disconnected. This ensures your safety and prevents accidental engine starts.

2. Remove the Spark Plug: Use a spark plug socket and ratchet to remove the spark plug from the cylinder head. Carefully unscrew the spark plug in a counterclockwise direction.

3. Examine the Spark Plug: Once the spark plug is removed, inspect its condition. Look for signs of wear, such as a worn electrode or a cracked insulator. Additionally, check for carbon buildup or oil fouling, which can also affect the spark plug’s performance.

4. Measure the Spark Plug Gap: Use a spark plug gap tool to measure the distance between the center electrode and the ground electrode. Refer to the lawn mower’s manual for the correct gap specifications. If the gap is too wide or too narrow, it may indicate a faulty spark plug.

Replacing a Bad Spark Plug

1. Purchase a Replacement Spark Plug: Identify the correct spark plug for your lawn mower model by referring to the manual or consulting a professional. Purchase a new spark plug that matches the specifications.

2. Install the New Spark Plug: Apply a small amount of anti-seize compound to the spark plug threads to prevent seizing. Carefully thread the spark plug into the cylinder head by hand, ensuring it is properly seated. Use a spark plug socket and ratchet to tighten the spark plug snugly, but avoid overtightening.

3. Reconnect the Spark Plug Wire: Once the new spark plug is installed, reconnect the spark plug wire securely.


Recognizing the signs of a bad spark plug in a lawn mower is essential for maintaining optimal engine performance. Difficulty starting the engine, engine misfires, rough idle, and decreased fuel efficiency are all indicators of a faulty spark plug. By following the steps to check and replace the spark plug, you can ensure that your lawn mower continues to run smoothly and efficiently.