How To Test A Spark Plug On A Lawn Mower

If your lawn mower is hard to start, running rough, or just won’t run at all, a faulty spark plug is often the culprit. Learning how to test a spark plug on a lawn mower is a simple skill that can save you time and money, and this guide will walk you through it step-by-step.

You don’t need to be a mechanic. With a few basic tools and about 15 minutes, you can diagnose the problem yourself. Let’s get your mower running smoothly again.

Before you start testing, you need to safely remove the plug from your engine. This process is straightforward but requires a bit of care.

Step 1: Gather Your Tools and Ensure Safety

First, collect what you’ll need. You probably have most of this in your garage already.

A spark plug socket wrench (usually 13/16″ or 5/8″)
A standard socket wrench or ratchet
A pair of insulated pliers or rubber gloves
A new spark plug for testing (optional but helpful)
A clean rag

Safety is crucial. Always disconnect the spark plug wire before you begin. Then, move the mower to a flat, dry surface and allow the engine to cool completely. You don’t want any accidental starts or burns.

Step 2: Locate and Remove the Spark Plug

Find the spark plug wire. It’s a thick rubber boot attached to a short, thick cable, usually coming from the engine’s front or top. Firmly grasp the boot (not the cable) and twist it gently while pulling it straight off.

Now, use your spark plug socket and ratchet. Fit the socket over the plug, turn it counter-clockwise to loosen it, and then unscrew it by hand. Be careful not to tilt it; you want to keep the threads in good shape.

Step 3: The Visual Inspection

Before any fancy tests, your eyes can tell you a lot. Wipe the plug clean with your rag and examine it closely.

Normal Wear: The electrode (the small metal rod at the tip) may be slightly worn, and the insulator (the white ceramic part) might be light tan or gray. This is okay.
Carbon Fouled (Black & Sooty): This indicates a rich fuel mixture, a dirty air filter, or too much idling.
Oil Fouled (Wet & Black): Oil is getting into the combustion chamber, which could signal worn engine parts.
Burnt or Melted Electrode: This points to overheating, which can be caused by incorrect fuel mixture or engine timing issues.
Heavy Deposits: Chalky or rust-colored deposits can come from fuel additives or a worn engine.

If the plug shows any signs of oil, heavy deposits, or is physically damaged, it’s best to just replace it. A visual check often gives you your answer right away.

Adjusting the Gap While You’re At It

If the plug looks okay, check the gap. This is the space between the center and side electrode. Use a feeler gauge. Most mowers need a gap between 0.028″ and 0.032″, but check your manual.

Slide the correct gauge between the electrodes. If it’s too wide, gently tap the side electrode on a hard surface to close it. If it’s too narrow, carefully pry it open with the gauge tool. Don’t touch the center electrode, as it’s very fragile.

Step 4: Performing the Spark Test

This is the definitive test to see if the plug can create a spark. There are two common methods.

Method 1: The Ground Test (Safest and Easiest)

Reconnect the spark plug wire to the plug you just removed.
Using your insulated pliers or wearing rubber gloves, hold the plug by the boot or the ceramic insulator only. Do not touch the metal part.
Position the metal threaded body of the plug against a clean, unpainted metal part of the engine block.
Make sure the plug’s electrode is not touching anything else.
Now, try to start the engine (pull the cord or turn the key).

What to Look For: You should see a bright blue spark jump across the electrode gap. A faint orange or yellow spark, or no spark at all, means the plug is bad. If you get a strong blue spark, the plug is likely good, and your problem lies elsewhere.

Method 2: Using a Spark Plug Tester (Most Reliable)

An inexpensive inline spark tester is a great tool. It looks like a clear plastic plug with a light inside.

Disconnect the spark plug wire from the original plug.
Connect the tester between the spark plug wire and the original plug, or connect it directly to the wire and ground its clip to the engine.
Try to start the engine.

The light in the tester will flash with each spark. A bright, consistent flash means good spark. A weak or no flash confirms a problem with the ignition system—which could be the plug, the wire, or the coil.

Step 5: Interpreting Results and Next Steps

You’ve tested the plug. Here’s what your results mean and what to do next.

Strong Blue Spark: Your spark plug is working. If the mower still won’t start, the issue is likely fuel-related (old gas, clogged carburetor) or air-related (dirty air filter). Compression problems are rarer.

Weak or No Spark: First, try a brand new spark plug. Repeat the test with the new plug. If you now get a good spark, the old plug was the problem. If you still get no spark with a new plug, the issue is in the ignition system.

What if a New Plug Doesn’t Spark?

This means the problem is deeper. The ignition coil (which generates the spark) could be faulty. The flywheel key could be sheared, throwing off the timing. The spark plug wire itself could be damaged. Or, a safety switch (like the blade engagement switch) could be preventing spark. These diagnoses require a bit more mechanical know-how.

Step 6: Reinstalling or Replacing the Plug

If your old plug tested good and looks good, you can reinstall it. If it failed, put in a new one.

Thread the plug into the engine by hand to avoid cross-threading. This is very important.
Once it’s hand-tight, use your socket wrench to give it a final snug turn. Do not overtighten. Usually, a 1/4 to 1/2 turn past hand-tight is sufficient.
Reconnect the spark plug wire by pushing the boot firmly onto the plug until you feel or hear a click.

That’s it. Give the starter cord a pull or turn the key. With a bit of luck, your mower will roar back to life.

Preventative Maintenance Tips

Testing is great, but prevention is better. Follow these tips to avoid future problems.

Replace Annually: Change your spark plug every mowing season, or after about 25 hours of use. It’s a cheap and effective habit.
Use Fresh Fuel: Old, stale gasoline is a top cause of engine issues. Use a fuel stabilizer if you store your mower for the winter.
Keep it Clean: Regularly clean around the spark plug area to prevent debris from falling into the cylinder when you remove the plug.
Check Your Air Filter: A dirty air filter causes a rich fuel mixture, leading to carbon fouling on the plug. Clean or replace it often.

FAQ Section

What does a bad lawn mower spark plug look like?
It will often be black and sooty (carbon fouled), wet with oil, or have a badly eroded or melted electrode. Any of these signs mean it should be replaced.

Can you clean a fouled spark plug?
You can for minor carbon fouling. Use a wire brush and some carburetor cleaner. However, for oil fouling or severe deposits, replacement is always the better and safer option. A new plug is inexpensive.

How often should I test my lawn mower spark plug?
You really only need to test it if you’re experiencing problems. Otherwise, just replace it once a year as part of your seasonal tune-up. It’s a simple task that keeps your mower reliable.

Why does my plug have spark but the mower won’t start?
Spark is just one part of the equation. You also need fuel and air. The most common causes are stale gasoline, a clogged carburetor jet, or a choked air filter. Always check these simple things first after confirming you have a good spark.

Is it safe to test a spark plug by holding it?
You should never hold the metal part with your bare hands during a ground test. The voltage is very high and can deliver a painful shock. Always use insulated pliers or wear protective rubber gloves to be safe.

By following this guide, you’ve learned a valuable skill that extends beyond just lawn mowers. The same basic principles apply to many small engines, from string trimmers to snow blowers. Remember to always put safety first, disconnect the wire before you start, and take your time. A well-maintained spark plug is a small part that makes a huge difference in getting your yard work done without any hassle.