Walk-Behind Lawn Mower: Won't spark
Check your spark plug because a fouled or cracked spark plug won't spark to ignite the fuel in the engine cylinder. A defective ignition coil will also prevent the spark plug from sparking. Use a spark plug tester to check the ignition coil. Replace the ignition coil if it doesn't send electric current to the spark plug.
Troubleshooting video: Lawn Mower Won’t Start: Spark Plug and Ignition Problems.
These repairs may help solve your Walk-Behind Lawn Mower problem:
Replace a lawn mower flywheel key
The flywheel key is a small metal rectangle that keeps the crankshaft and flywheel aligned when you tighten the flywheel nut. To protect more expensive components from damage, the flywheel key is designed to shear if the mower blade hits an object like a rock or tree stump hard enough to make the flywheel slip out of alignment with the crankshaft. If the flywheel shear slightly, the engine runs rough or misfires. If the flywheel key breaks or shears severely, your lawn mower stops running and won’t start. If the flywheel key breaks or shears, replace it.
Replace a lawn mower safety switch
The engine safety switch on a walk-behind lawn mower stops the ignition coil from sparking and has a brake pad that slows the flywheel. If the safety switch fails, the engine might not turn off (in that case, remove the spark plug wire to kill the engine). A failed switch prevents the ignition coil from sparking the spark plug. If the coil isn't sparking, remove the kill wire from the ignition coil and check the engine for spark. If there's spark, replace the safety switch.
Replace a lawn mower ignition coil
The ignition coil sits next to the flywheel and generates electrical current to fire the spark plug when magnets on the flywheel pass the ignition coil. If the ignition coil fails, the engine won't start because the spark plug doesn't fire. Connect a spark plug tester to the engine to see if the spark plug is getting electrical current from the ignition coil. If the ignition coil doesn't work, replace it.
Replace a lawn mower spark plug
The spark plug gets electrical current from the ignition coils and sparks to ignite the fuel mixture in the cylinder, which drives the piston. The spark plug won't spark if it's coated with carbon deposits or oil residue, or if there’s a crack in the ceramic insulator. The engine won't start if the spark plug is bad. Replace the spark plug if it's damaged or coated with residue.