Mower Won't Start No Spark (This Is Why)

By: Author John Cunningham. Published: 2019/11/05 at 7:30 am

Pulling and pulling and nothing; a mower without a spark is useless. In this post, we’ll cover all the most common ignition system failures.

Mower won’t start–no spark? Common reasons a lawnmower has no spark include:

Plug fault
Plug wire fault
Stop/start switch fault
Coil (Armature) Fault

None of these tests is difficult, and in twenty minutes from now, you’ll know why your mower has no spark. This post will have you covered, but if you need video help diagnosing no spark or help fit a new coil, check out the “Mower won’t start video.”

Checking Lawnmower Spark

Since you’ve already checked the spark, I’m guessing you know the procedure. However, it’s worth noting that getting this test wrong can lead to a misdiagnosis and unnecessary replacement of the ignition coil or other parts.

Spark testing is, as you know, a simple test. You won’t need any special tools here, but a spark-testing tool makes the job easier and totally foolproof.

If you need video help, check out the mower “Mower spark test video,” where I cover the whole process.

Tools needed

For these tests, you’ll need a plug spanner, insulated pliers, screwdrivers, and a spark plug. You’ll also need a helper, as we’re not using a spark testing tool. It can be difficult to crank the engine and check for spark at the same time. With all the tools gathered and a helper on hand, we’ll get right to it.

Spark Testing

Since we’ll have a helper cranking the engine, the blade will be spinning, and even though the engine’s not running, it can still remove body parts, so you know!

You must use insulated pliers (plastic- or rubber-handled) to hold the plug, as the voltages produced can give you a jolt, which isn’t pleasant.

Tools – A plug spanner, insulated pliers, and a spark plug will be needed.

Spark test tools

Step 1 – Remove the spark plug wire by twisting and pulling, then use the plug tool to remove the spark plug.

Step 2 – Reattach the spark plug wire to the plug. Using your insulated pliers, hold the plug threads firmly against the engine’s metal. This is known as grounding. If the plug doesn’t make good contact with the engine’s bare (unpainted) metal, you won’t get a spark.

Step 3 – While you watch for spark, have the helper hold the bail lever as normal and yank on the pull cord.

If you have no spark, swap out the plug and test again.

If you still have no spark, it is most likely a failed coil, but it is best to check the on/off switch assembly first.

Common Spark Plug Faults

A healthy spark plug is essential for reliability, power, and smooth running. Plugs have a tough job. They carry high voltages and live at the heart of the engine where it’s hottest.

Making matters worse for the plug is its location: right out in front of the engine. So, getting shoved into fences and trees is all part of a spark plug’s life, and you thought you had it hard!
These are the most common spark plug faults:

Wrong plug type
Dirty plug
Bad plug gap
Cracked spark plug insulator

Wrong Plug Type

Plug as you know are graded; each engine will have a particular plug code. So even though a plug fits, it doesn’t mean it’s correct. Plugs are graded by heat. The plug should heat enough to burn off contaminants but not so much that it pre-ignites. Wrong plug types can cause all kinds of problems, from hard starting and rough running to hot-start failures.

Plug type – Check your plug type with your mower engine maker.

An incorrect plug type will lead to intermittent problems.

A New Spark Plug Isn’t Always a Good Spark Plug

It’s tempting to assume a brand-new spark plug must be working correctly, but that’s not always true.

I’ve seen new plugs dropped on concrete floors, damaged during installation, or simply faulty right out of the box.

If you’re chasing a no-spark condition, don’t automatically rule out a new plug. Testing with a second known-good plug is often the quickest way to eliminate any doubt.

Dirty Plug

Self-explanatory, it’s a plug that’s contaminated by too much gas (flooding), carbon, or oil. All of these will prevent the plug from doing its job. Flooding may be caused by a few reasons: a blocked air filter, a faulty choke, overuse of the choke, tipping the mower over on its carburettor side, or a carburettor fault. Check out the video “How to fix a flooded engine.”

Carbon build-up in the engine is a normal condition. Fuel type, oil type, maintenance, and plug type all affect how quickly it builds.

Oil on the plug is also common. It’s caused by too much oil, a blocked crankcase breather, a head gasket fault, engine wear, and the wrong plug type. Check out the video “How to clean a plug.”

Bad Plug Gap

A spark plug function is obviously to create a spark, and it can only do this if the electrode gap is correct. The coil has been designed to create a sufficient spark to jump a pre-determined spark plug gap.

No gap, means no spark
Gap too small means poor running or no start
Gap too big means no start and risks damaging the coil

A plug gap tool is used to set the spark plug gap. The electrode is manipulated to the correct size by simply bending it with pliers. Check out the video “How to gap a plug.”

Plug gap – The gap is important. Too small or too big can lead to no starts or poor running.

Cracked Plug Insulator

Self-explanatory too. The insulator is the white ceramic material in the plug’s body, and, as mentioned earlier, plugs are at risk of damage from bumping into obstacles. If the insulator breaks or cracks, the plug stops working.

Common Spark Plug Wire Faults

A spark plug wire has a few specific problems that depend on variables like how and where it’s stored.

The common faults I see again and again include:

Loose terminal connector
Faulty terminal connector
Damaged plug wire

Loose Terminal

Caused by our old friends, the trees, shrubs, and fences. The plug wire terminal that clips to the spark plug can become loose, causing no starts, poor running, or intermittent starting/running.
The fix here is simple: squeeze the terminal body using pliers to tighten it.

A loose terminal will cause the engine to misfire or not start at all. The quick fix here is to squeeze the terminal until it fits snugly on the plug.

Faulty Terminal

Because this cap was loose, it caused arching, which burned the terminal cap.

Faulty terminal connector – It’s different but related to a loose connector. A loose connector will often become faulty as the spark starts to jump inside the terminal, burning it or setting the stage for corrosion to take hold.

The outcome is the same: no spark or poor running. A replacement terminal can be purchased and fitted to solve this issue.

Damaged Plug Wire

A plug wire rubbing against the engine cover can wear the insulation and cause the coil to ground. But more often than not, a damaged plug wire means rodents. Mice love wiring insulation, and unfortunately, our furry friends have cost us a coil.

Sure, you can wrap them with insulation tape, but it’s only a quick fix. The long-term repair is to replace.

Damaged wire – Mice love to chew on the wiring insulation.

Winter Storage and Rodent Damage

If your mower ran perfectly last season but suddenly has no spark after winter storage, rodents should be high on your suspect list.

Mice love warm engine compartments and frequently chew ignition wiring while building nests beneath engine covers.

Remove the engine shroud and inspect carefully for:

Chewed wires
Nesting material
Acorn storage
Damaged insulation

I’ve found plenty of no-spark faults caused by mice that took less than five minutes to diagnose once the cover came off.

Common Stop/Start Assembly Faults

Most mower owners are familiar with the bail lever at the handlebars, which must be held to start the mower. Most mowers will use this type of stop/start system; other manufacturers may incorporate the stop/start function with the throttle lever. But apart from this difference, all other components will be very similar.

If you’re working on a ride-on mower, don’t forget about the safety switch system.

Many modern lawn tractors use several safety switches that can prevent spark if they fail or become disconnected. Common switches include:

Seat switch
Blade engagement (PTO) switch
Brake pedal switch
Reverse operation switch

A faulty safety switch can mimic a bad ignition coil because the engine may crank normally but never produce spark.

If your mower suddenly lost spark after washing, servicing, or storing it over winter, inspect the safety switch connectors for corrosion, loose terminals, or damaged wiring before replacing expensive ignition components.

Anyhow, the main components of the stop/start assembly include:

Bail/throttle lever
Cable
Flywheel brake assembly
Stop/start switch
Coil control wire

Bail / Throttle Lever

Common faults here include disconnected, out-of-adjustment, or broken levers.

Cable

The cables break and stretch, so it’s not uncommon for the bail lever to work, but because the cable has stretched, it doesn’t move the brake assembly to the start position.

Stop/start cable

Flywheel Brake Assembly

Common faults here include a cable out of adjustment, meaning the bail lever doesn’t pull the brake to the off position.

Flywheel assembly

Stop/Start Switch

This is the on/off switch. It’s fitted at the flywheel brake assembly. When the bail lever pulls the assembly, it pushes on the switch, removing the ground connection to the coil. This allows the mower to start.

On /off switch

Coil control – Here’s a different mower coil control switch. It’s a very simple connection; the contact points must separate before the coil and plug will create a spark.

The Coil (also known as Armature)

The control wire connects the stop/start switch on the flywheel brake assembly to the coil, which is fitted to the engine. The coil and plug won’t produce a spark so long as the control wire is connected to ground (the engine’s metal).

Coil

A common fault is the chafing of the control wire on the engine (shorting to the ground); this effect is the same as releasing the bail lever – turns the engine off.

Check the coil control wire for chafing, especially anywhere the wiring turns sharply around the engine.

Coil control wire: Coil control is a single-wire system with a push-on connection. Often, they’ll come loose, and when they do, the mower won’t turn off.

Quick Coil Test

Before replacing the ignition coil, disconnect the coil control wire.

This wire grounds the coil when the stop switch is activated.

With the wire disconnected:

Remove the spark plug.
Reconnect the plug wire.
Ground the plug against the engine.
Pull the starter rope.

If the spark returns with the control wire disconnected, the coil is usually good, and the fault lies somewhere in the stop/start system.

If there is still no spark, the ignition coil is the most likely culprit.

Common Coil Faults

Coils generally work, or they don’t. Occasionally, you’ll get a coil that works when it’s cold and stops when the engine heats up. Coils are solid-state units – they can’t be repaired. Testing a coil and fitting a new one is easy; I wrote a whole post about it right here “Push mower hard to start when hot.”

Check out the video here; it covers spark checking, diagnosing, and replacing the coil. If you need to replace the coil, check out the great deals on the Amazon link below.

Amazon Lawnmower Coils