How To Tell If A Lawn Mower Spark Plug Is Bad

When your lawn mower refuses to start or runs rough, the problem is often a simple one. Knowing how to tell if a lawn mower spark plug is bad can save you time, money, and a lot of frustration. This guide will walk you through the clear signs and easy checks you can do yourself.

You don’t need to be a mechanic. With a few basic tools and some visual inspection, you can diagnose a faulty plug in minutes.

A spark plug is a small but critical part. It creates the spark that ignites the fuel in your engine’s cylinder. When it fails, the engine can’t run properly. Let’s look at the most common symptoms that point to a bad plug.

Clear Signs Your Spark Plug Needs Attention

Before you even pick up a tool, your mower’s behavior often tells the story. Watch for these issues:

Hard Starting or No Start: This is the most obvious sign. You pull the cord repeatedly, but the engine just won’t catch. Without a good spark, there’s no ignition.
Rough Idling and Misfiring: The engine starts but sounds uneven or shaky. It might surge up and down on its own or feel like it’s stumbling.
Poor Engine Performance: You notice a significant loss of power. The mower struggles in thick grass it used to handle easily.
High Fuel Consumption: An incomplete burn due to a weak spark means you’re using more gas to do the same work.
Excessive Exhaust Smoke: Unburned fuel can exit as black or dark smoke from the muffler.

What You’ll Need for Inspection

Gathering your tools first makes the job smoother. You likely have most of these already:

A spark plug socket wrench (usually 13/16″ or 5/8″)
A pair of insulated pliers or a spark plug tester (optional but helpful)
A clean rag
A small wire brush or piece of emery cloth
A gap tool (feeler gauge) for checking the electrode spacing
A new spark plug (check your mower’s manual for the correct type)

Step-by-Step: How to Remove and Inspect the Plug

Always ensure the mower is off and completely cool. Disconnect the spark plug wire for safety before you begin.

Step 1: Locate and Disconnect the Plug

Find the spark plug. It’s a short, porcelain-covered component on the side of the engine cylinder, with a thick rubber wire attached to its top. Firmly grasp the rubber boot and pull it straight off. Don’t pull on the wire itself.

Step 2: Remove the Plug

Fit your socket wrench over the plug. Turn it counterclockwise to loosen and unscrew it. Once loose, you can finish unscrewing it by hand. Try to keep it upright so any debris doesn’t fall into the open engine cylinder.

Step 3: The Visual Inspection – Reading the Plug’s Condition

This is where you learn how to tell if a lawn mower spark plug is bad by looking at it. Hold the plug and examine the tip (the electrode end) closely.

Normal Wear: The insulator tip should be a light tan or gray color. The electrodes will show some erosion but are still distinct. This plug is likely fine but may be near the end of its service life.
Carbon Fouled (Dry, Sooty Black): A fluffy black deposit indicates a rich fuel mixture, weak ignition, or too much idling. This can prevent a spark.
Oil Fouled (Wet, Shiny Black): Wet, oily deposits signal oil is leaking into the combustion chamber. This is a more serious engine issue, but it starts by ruining the plug.
Burnt or Blistered Electrode: A white, blistered insulator or severely eroded electrodes mean the plug has been running too hot. This is often due to incorrect plug type or engine timing problems.
Heavy Deposits or Corrosion: Crusty buildup can bridge the gap, shorting out the plug. This is common from fuel additives or prolonged use.
Cracked Insulator or Broken Electrode: Any physical damage means the plug is definitively bad and must be replaced immediately.

The Spark Test: A Definitive Check

If the plug looks okay visually, a spark test will confirm its function. Warning: This involves creating a visible spark. Ensure no fuel or flammable vapors are present.

Reconnect the spark plug wire to the plug you removed.
Using insulated pliers, hold the plug by the rubber boot so your hand is not touching the metal.
Touch the threaded metal body of the plug to a clean, unpainted metal part of the engine block.
Have a helper pull the starter cord (or use your electric start) while you watch the electrode gap.
You should see a bright, blue spark jump across the gap. A weak orange spark or no spark at all confirms a bad plug or ignition system issue.

For a safer method, invest in an in-line spark plug tester from any auto parts store. It lights up when spark is present.

Checking and Setting the Electrode Gap

Even a good-looking plug can fail if the gap is wrong. The gap is the space between the center and side electrode.

Find the correct gap specification in your mower manual (common gaps are .028″ to .035″).
Use your gap tool to measure the existing space. Slide the correct-sized blade through; it should have a slight drag.
If adjustment is needed, carefully bend the side electrode only a tiny bit using the gap tool’s bending arm. Re-measure until it’s correct.

Never try to adjust the gap by banging the plug on a surface, as this can damage the internal insulator.

Cleaning vs. Replacing: What’s Best?

For a lightly carbon-fouled plug, you can sometimes clean it. Use a wire brush to gently clean the electrode tip and threads. You can also use a bit of emery cloth to clean the electrodes.

However, spark plugs are inexpensive consumable parts. Cleaning is a temporary fix at best. For reliable performance, replacement is almost always the better choice, especially if the plug is a few years old.

How to Install a New Spark Plug

Check the gap on your new plug and adjust if necessary before installation.
Carefully thread the new plug into the engine hole by hand to avoid cross-threading. It should spin in easily.
Once hand-tight, use the socket wrench to give it a final snug turn—about 1/8 to 1/4 turn more. Do not overtighten.
Push the rubber boot back onto the top of the plug until you feel or hear it click into place.

Your mower should now start easier and run smoother. If it doesn’t, the problem may lie elsewhere, like old fuel, a dirty air filter, or carburetor issues.

FAQ: Common Spark Plug Questions

How often should I change my lawn mower spark plug?

As a general rule, replace it once per mowing season. For heavy use, consider changing it mid-season. It’s a cheap part that makes a big difference.

Can a bad spark plug ruin a lawn mower engine?

While a single bad plug won’t typically cause permanent damage, running with a fouled plug for a long time can lead to excessive fuel consumption and carbon buildup. A physically damaged plug with a broken electrode can cause severe engine damage if pieces fall into the cylinder.

What causes a spark plug to go bad quickly?

Frequent short runs (mowing for only 10-15 minutes) don’t let the engine get hot enough to burn off deposits. Using old gasoline, a dirty air filter, or an incorrect fuel-oil mix in 2-stroke engines are also common culprits.

Are all lawn mower spark plugs the same?

No. They vary by thread size, reach, heat range, and electrode design. Always use the plug type recommended in your owner’s manual for best results. Using the wrong one can cause poor performance or engine damage.

My plug is wet after trying to start. What does that mean?

A wet plug usually means the cylinder is flooded with fuel. This happens after many unsuccessful start attempts. Dry the plug thoroughly with a rag, let the cylinder air out for 15 minutes, and try again with the choke set correctly.

Final Tips for a Healthy Mower

Regular maintenance prevents most spark plug issues. Always use fresh fuel, stabilize gas over the winter, and keep your air filter clean. At the end of each season, remove the plug, add a teaspoon of oil into the cylinder, and pull the cord slowly to coat the cylinder walls before storage. This prevents corrosion.

Learning how to tell if a lawn mower spark plug is bad is a fundamental skill for any gardener. It’s a quick, satisfying repair that gets your equipment running right and keeps your lawn looking its best. With this guide, you’ll never be stumped by a reluctant mower again.