Lawn Mower Not Getting Spark: 2026 Troubleshooting

A lawn mower not getting spark is one of the most frustrating starting problems you can face at the start of the season — and in 2026, it remains just as common across every major engine brand. Some fixes take 15 minutes and a $6 part; others require disassembling the top of the engine. This guide ranks the four causes from most likely to least likely so you can diagnose the problem efficiently without replacing parts you don’t need.

4 Common Causes for a Lawn Mower Not Getting Spark

1Worn or Fouled Spark Plug

The spark plug is the single most common reason a lawn mower loses spark, and it is always the right place to start. Over time, carbon deposits build up on the electrode, the electrode itself erodes, and the ceramic insulator can crack — any of which prevents the plug from firing reliably. A plug gap that has widened beyond your engine’s specification produces a weak or intermittent spark that may not ignite the fuel mixture at all.

Symptoms

• Engine cranks normally but will not fire or fires very inconsistently
• Visible black carbon fouling, a cracked insulator, or a rounded, eroded center electrode when the plug is removed
• Weak orange or yellow spark (instead of a sharp blue one) during a bench test

Care Plan

1. Allow the engine to cool for at least 15–20 minutes if a recent start attempt was made, then disconnect the spark plug wire by pulling the rubber boot (the cap at the end of the wire) straight off the plug — pull the boot itself, not the wire.
2. Use a spark plug socket (typically 5/8” or 13/16”) to remove the plug. Inspect the electrode for heavy carbon buildup, cracks in the white ceramic insulator, or visible erosion of the center electrode tip.
3. With the wire reconnected to the plug, hold the plug’s threaded metal body firmly against a bare metal section of the engine block and pull the recoil starter. A healthy plug produces a sharp, bright blue spark. A weak orange spark or no spark confirms the plug is suspect. Before cranking, ensure the area around the open plug hole is clear of raw fuel or fuel vapour — do not perform this test immediately after a flooded start attempt.
4. Replace the plug with the manufacturer-specified part number (found in your owner’s manual or printed on the old plug). Set the electrode gap using a feeler gauge to the specification listed in your engine’s owner’s manual — do not use a universal range as a substitute, since Honda GCV engines commonly specify 0.028”–0.031”, many Briggs & Stratton engines specify 0.030”–0.035”, and some Kohler engines specify as little as 0.020”. Using the wrong gap for your specific engine can result in weak or no spark. If you own a twin-cylinder engine (common on riding mowers), replace both plugs at the same time.
5. Install the new plug finger-tight first, then tighten it an additional 1/4 to 1/2 turn with the socket wrench — this is the standard range for plugs installed into aluminum cylinder heads and prevents both under-tightening (which causes misfires) and over-tightening (which can strip the threads). Reconnect the boot and test. If the mower still will not produce spark after a confirmed new plug at the correct gap, move to Cause 2.

Common Mistakes

• Gapping the plug to a generic “standard” specification rather than the value listed in your specific engine’s owner’s manual — this is one of the most common causes of a no-spark condition after a plug change.
• Reusing a plug that is visibly fouled or cracked rather than replacing it, on the assumption that cleaning it will restore full performance.

2Failed Ignition Coil

The ignition coil (also called the armature) converts battery or flywheel-generated current into the high-voltage pulse that fires the spark plug. Coils fail from heat cycling over many seasons, from impact damage, or from internal winding breakdown. Because the coil sits very close to the spinning flywheel magnets, even minor physical damage can cause complete failure. A faulty coil is the second most likely cause of a lawn mower not getting spark once a new plug has been ruled out.

Symptoms

• No spark at all even after installing a confirmed new, correctly gapped plug
• Mower starts when cold but loses spark and dies after several minutes of running, then restarts only after a long cool-down period (classic sign of a heat-sensitive coil)
• Visible cracking, melting, or physical damage to the coil body when inspected

Care Plan

1. Disconnect the spark plug wire boot and remove the spark plug. Access the ignition coil — on most walk-behind mowers it is mounted on the side of the engine block adjacent to the flywheel and is secured by two bolts.
2. Locate the kill wire: a single small wire (often red, green, or bare copper) that connects from the coil’s grounding terminal to the throttle/safety kill switch circuit. Before proceeding to step 3, note this important safety condition: with the kill wire disconnected in step 3, the only way to stop the engine if it fires is to release the bail lever or physically choke the fuel supply. Ensure the blade engagement lever is fully disengaged, the mower is on a flat hard surface, and no bystanders are near the blade before continuing.
3. Carefully disconnect the kill wire from the ignition coil terminal. With the spark plug wire reattached to the plug and the plug’s threaded body held firmly against a bare metal section of the engine block, pull the recoil starter. If spark now appears with the kill wire disconnected, the coil itself is functional — the fault is in the kill circuit (proceed to Cause 3). If there is still no spark, the coil is the likely failure. Do not run the engine with the kill wire permanently bypassed — reconnect it before any normal operation.
4. To test the coil more formally: set a multimeter to resistance (Ω) mode and measure primary resistance (between the kill wire terminal and the coil’s metal lamination/core). Note that resistance specifications vary significantly by manufacturer and coil type — consult your engine’s service manual for the specific value. Important: many modern small-engine ignition systems use CDI (capacitor discharge ignition) rather than inductive ignition. CDI coils often cannot be reliably diagnosed by resistance testing alone because a standard multimeter may show a plausible reading on a coil with a failed trigger circuit. If your engine uses a CDI system, use a dedicated inline spark tester tool for a more reliable result. If the coil fails the resistance test or produces no spark with a tester, replace it.
5. Install the replacement coil, setting the air gap between the coil’s legs and the flywheel magnets using a business card or the feeler gauge dimension specified in your engine manual (typically 0.010”–0.014” for most Briggs & Stratton engines — confirm your spec). Tighten the mounting bolts, then remove the feeler gauge or business card completely before cranking the engine — leaving it in place can eject it at speed or damage the coil. Reconnect the kill wire, reinstall the spark plug, and test. If spark is restored but the mower still will not start, the carburetor or compression may be a secondary issue — consider escalating to a professional. Note that walk-behind mower aftermarket coils typically cost $20–$60; OEM coils for Kawasaki or Kohler riding mower engines can run $80–$150 or more. Prices also vary by region.

Common Mistakes

• Deciding the coil has failed without first confirming the spark plug is new and correctly gapped — always eliminate the plug as a cause before concluding the coil needs to be replaced.
• On CDI-equipped engines, relying solely on a multimeter resistance test and passing a coil that has actually failed — use a spark tester tool for a definitive result on CDI systems.

3Faulty Kill Switch or Kill Wire

Every lawn mower has a kill circuit — a system of switches and a grounding wire that shuts the ignition coil off when you release the bail lever, turn the key off, or trigger a safety interlock. When any part of this circuit shorts to ground continuously, the coil is permanently suppressed and the engine will never fire. Walk-behind mowers typically use a single bail-operated kill wire; riding mowers add seat switches, blade engagement interlocks, and brake/clutch switches — any of which can fail in the “open” or “shorted” position.

Symptoms

• No spark with a known-good, correctly gapped plug and a functional coil (confirmed by the kill wire disconnect test in Cause 2)
• Spark returns when the kill wire is disconnected from the coil terminal
• Intermittent starting that varies depending on how the bail handle, seat, or safety levers are positioned

Care Plan

1. With the engine cold and the spark plug wire disconnected, visually trace the kill wire from the ignition coil terminal back toward the handlebar bail lever or key switch. Look for visible chafing, pinching against the engine block, or a bare wire touching a metal frame surface.
2. Disconnect the kill wire at the coil terminal and use a multimeter set to continuity mode. With the bail lever fully released (kill position), you should read continuity (the circuit is closed to ground — this is normal and correct). With the bail lever fully engaged (run position), you should read no continuity. If you read continuity in both positions, the switch or wire is stuck in the kill state and is the fault.
3. On riding mowers, engage the parking brake, work on level ground, and chock the wheels before accessing any under-seat or underside components. Locate the seat safety switch under Locate the seat safety switch under the mower seat. Disconnect the wiring harness and test the switch for continuity. The switch should change state (open to closed) when the plunger is depressed. If the switch is stuck or the harness is corroded, the ignition will remain grounded, preventing spark.
4. Inspect the blade engagement (PTO) switch. On riding mowers, if the blades are “engaged” (or the switch thinks they are), the engine will not spark. Verify the lever is fully disengaged and the switch is clicking.
5. Replace or Repair: If a specific switch is failed, order an OEM replacement ($10–$30). If the wire itself is chafed, wrap the exposed section in electrical tape or replace the wire run to prevent it from grounding out against the mower frame. Reconnect the kill wire to the coil once the fault is cleared.

Common Mistakes

• Permanently bypassing safety switches. Never zip-tie a bail handle or jump a seat switch permanently. These are critical safety features designed to stop the blades if you fall or let go; bypassing them creates a severe injury risk.
• Assuming a bad coil when the wire is just pinched. A kill wire pinched under a blower housing bolt is a free fix that many homeowners mistake for a $50 coil failure.

4Sheared Flywheel Key

The flywheel key is a small, soft metal tab that aligns the flywheel with the crankshaft. It is designed to “shear” (break) if the mower blade hits a solid object like a rock or stump. This acts as a fuse to protect the crankshaft from bending. When it shears, the flywheel shifts out of alignment, which throws off the ignition timing. While the coil may still produce a spark, it won’t happen at the right time in the combustion cycle, making it appear as if the mower has “lost its spark” or won’t start.

Symptoms

• The mower kicked back or stopped suddenly after hitting an object.
• The engine has spark (confirmed with a tester), but it won’t start or backfires loudly.
• The starter rope pulls back violently or feels “hard” to pull at certain points.

Care Plan

1. Access the Flywheel: Remove the starter recoil housing and the blower fan shroud from the top of the engine.
2. Inspect the Keyway: Look at the center of the flywheel where it meets the crankshaft. The slot (keyway) in the flywheel should line up perfectly with the slot in the crankshaft. If they are offset by even a millimeter, the key is sheared.
3. Remove the Flywheel: You will need a large socket to remove the flywheel nut (use a block of wood to jam the blade or a strap wrench to hold the flywheel). Use a flywheel puller to pop the flywheel off the tapered crankshaft. Warning: Do not strike the crankshaft with a hammer, as this can destroy the engine’s internal bearings.
4. Replace the Key: Remove the broken pieces of the old key and slide a new one ($5) into the slot. Reinstall the flywheel and torque the nut to the specific foot-pounds listed in your engine manual (this is critical—if the nut is loose, the new key will shear immediately).
5. Test: Reassemble the shroud and recoil. If the timing was the issue, the engine should now fire up on the first or second pull.

Common Mistakes

• Using a steel “homemade” key. Flywheel keys are made of soft aluminum for a reason. If you use a hard steel key and hit another rock, you won’t shear the key—you will snap the crankshaft, which totals the engine.
• Failing to torque the flywheel nut. If the nut isn’t tightened to spec (often 50–65 ft-lbs), the inertia of the flywheel during the first start attempt will shear the brand-new key instantly.