Lawn Mower Troubleshooting: Won't Start
Created by Cheli
Step-by-step checklist to diagnose and fix a lawn mower that won't start.
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Checklist Items (23)
Prepare for safe troubleshooting
Gather safety gear, ensure mower is off and spark plug wire disconnected.
Wear safety glasses and gloves
Protect eyes from debris and hands from sharp edges.
Disconnect spark plug wire to prevent accidental start
Pull the rubber boot off the spark plug to prevent accidental ignition.
Inspect fuel system
Check fuel level, freshness, and flow.
Check fuel level in tank
Open fuel tank cap and visually confirm adequate fuel.
Verify fuel is fresh (<30 days) and free of water/debris
Fuel older than 30 days can cause starting issues; drain and refill if stale.
Inspect fuel lines for cracks, leaks, or blockages
Look for cracks, splits, or blockages; replace damaged lines.
Check and clean or replace fuel filter if equipped
If filter is clogged, replace it; otherwise clean with solvent.
Check ignition components
Verify spark plug and related parts produce spark.
Remove spark plug and inspect for fouling or damage
Use a spark plug socket to extract the plug for inspection.
Clean spark plug with wire brush or replace if worn
Use a wire brush to remove carbon deposits; replace if electrode worn.
Check spark plug gap with feeler gauge (0.020‑0.030 in)
Insert feeler gauge; adjust gap to manufacturer spec (usually 0.020‑0.030 in).
Reconnect plug wire and test for spark by grounding against engine
Hold plug threaded against engine block; pull cord and watch for spark.
Examine air intake and filter
Ensure engine receives adequate airflow.
Remove air filter cover and inspect filter condition
Unscrew or unclip cover to access filter.
Clean filter by tapping out debris; replace if excessively dirty
Tap out loose dirt; wash foam filter with soap and water if applicable, then dry.
Check air intake hose for obstructions, cracks, or loose connections
Ensure hose is securely attached and free of debris or cracks.
Check mechanical parts that could prevent engine turnover
Blade, belt, and engine seizure.
Disengage blade (if equipped) and ensure it rotates freely
Move blade engage lever to disengaged position; manually spin blade.
Inspect drive belt for wear, cracks, or breakage
Look for fraying, cracking, or missing sections; replace if damaged.
Attempt to manually turn engine crankshaft to check for seizure
Using a socket on the flywheel nut, try to rotate engine; it should turn smoothly.
Test electrical starting system
Only needed for models with electric start.
Check battery voltage with multimeter (~12.6V fully charged)
Set multimeter to DC volts; place probes on battery terminals.
Inspect battery terminals for corrosion; clean if needed
Clean corrosion with baking soda solution and a wire brush.
Verify safety switches (seat, blade engagement) are closed
Ensure seat switch is pressed when operator present and blade lever is disengaged.
Reassemble and attempt to start
After checks, try to start the mower.
Reconnect spark plug wire securely
Press the rubber boot firmly onto the spark plug tip.
Set throttle to appropriate position (fast/choke for cold start)
For cold engine, set throttle to choke; for warm engine, set to fast.
Engage any required safety levers (e.g., handlebar brake)
Hold handlebar brake lever against handle to complete safety circuit.
Pull starter cord briskly or press electric start button
Give a quick, firm pull; if electric start, turn key to start position.