Lawnmower Wont Turn Off

You’ve finished mowing, but your lawnmower wont turn off. That persistent engine trouble is more than annoying; it’s a safety hazard. Let’s figure out why it’s happening and how you can fix it safely. This guide will walk you through the common causes, from simple fixes to more complex issues, so you can get your mower back under control.

When the engine keeps running after you release the lever or turn the key, the problem is usually in the shutdown system. Your mower is designed to stop the engine by cutting off the spark or the fuel. If one of those paths fails, the mower keeps going. Don’t worry, we’ll start with the easiest checks first.

Common Reasons Your Mower Keeps Running

Several components can fail and cause this. Here are the most likely culprits:

Faulty Ignition Switch: On electric-start mowers, a bad switch won’t ground the ignition.
Stuck Blade Control Lever (Bail): The cable or mechanism that should disengage is jammed.
Damaged Wiring: A short or broken wire can bypass the off signal.
Carburetor Issues: A stuck throttle linkage or governor spring can keep fuel flowing.
Faulty Safety Interlock System: A misadjusted or broken safety switch interferes with shutdown.

Safety First: How to Stop the Engine Immediately

Before you investigate, you need to safely stop the mower. Never try to fix it while it’s running.

For a push mower, locate the spark plug wire. Carefully pull it off the plug. This stops the spark and kills the engine instantly.
On a riding mower, you can often stall it by engaging the brake and shifting into gear. As a last resort, locate the fuel shut-off valve and close it.
Once the engine is off, disconnect the spark plug for safety. This prevents any accidental starts while you’re working.

Checking the Blade Control Lever and Cable

This is the most common fix for push mowers. The lever you hold down to run the mower is connected to a cable. That cable pulls on the engine’s brake-clutch to stop the flywheel. If it’s stuck or out of adjustment, the engine won’t stop.

Look where the cable attaches to the engine. Is it moving freely when you engage and release the lever?
Spray a little lubricant on the cable ends and pivot points. Work the lever a few times.
Check for a broken or frayed cable. If it’s damaged, it will need replacement.

Inspecting the Ignition Switch

On riding mowers and electric-start models, the key switch is prime suspect. A faulty switch won’t complete the circuit that grounds out the ignition coil to stop the spark. You can test it with a multimeter, but a visual check for corrosion or loose wires is a good start. Sometimes the connections just come loose from vibration.

Understanding the Kill Wire and Grounding System

Most small engines stop by grounding the ignition coil. A wire runs from the coil to the ignition switch or blade control lever. When you turn the key to “off,” or release the lever, that wire touches a metal ground, shorting out the spark. If that wire is damaged, disconnected, or the grounding point is rusty, the engine can’t stop.

Trace the thin wire from the ignition coil on top of the engine.
Follow it to where it connects (to a safety switch or the blade lever mechanism).
Ensure the connection is tight and free of dirt or corrosion. Clean it with a wire brush if needed.

When the Carburetor is the Culprit

Less commonly, a engine that won’t stop points to fuel system issues. This is often called “dieseling” – the engine sputters and runs on from heat, not spark. It happens when the throttle plate is stuck open or the governor spring is loose, allowing extra fuel into a hot engine.

Check the throttle linkage on the carburetor. Make sure it moves smoothly and returns to the idle position.
Inspect the governor springs. Are they connected and intact? A broken spring means no speed control.
If you’re comfortable, removing and cleaning the carburetor can resolve a stuck throttle shaft.

Step-by-Step Troubleshooting Guide

Follow this logical order to find the problem without wasting time.

Disconnect the Spark Plug. Always start here for safety.
Check the Obvious. Inspect the blade control lever for free movement. Look for disconnected wires.
Test the Ignition Switch (if applicable). Use a multimeter for continuity or try a temporary replacement.
Inspect the Kill Wire. Trace it from the coil to its end point. Look for breaks or bad connections.
Examine the Carburetor Linkage. Manually move the throttle to see if it’s sticky or binding.
Review Safety Interlocks. On riding mowers, a faulty seat or brake switch can sometimes confuse the system.

What to Do About a Stuck Governor

The governor regulates engine speed. If its internal components are worn or the external arm is stuck, the engine may run at full throttle uncontrollably. This is a more advanced repair. You’ll need to remove the engine cover to access the governor gear inside the crankcase. For most gardeners, if the problem is internal governor failure, it might be time to consult a professional small engine mechanic.

Preventing the Problem in the Future

A little maintenance goes a long way. Here’s how to avoid this scary situation next season:

Always store your mower in a dry place to prevent corrosion on electrical parts.
Regularly clean the underside of the deck to prevent grass from jamming moving parts.
Lubricate control cable ends and pivot points once a season with a light oil.
Check wiring for rodent damage if the mower is stored in a shed over winter.
Use a fuel stabilizer if you store gas in the tank over the off-season to keep the carburetor clean.

FAQ: Lawn Mower Won’t Shut Off Problems

Q: Why does my lawn mower run on after I turn it off?
A: This “dieseling” is often caused by a carburetor issue. The engine gets hot enough to ignite fuel without a spark. Check for a stuck throttle or high idle speed.

Q: Can a bad spark plug cause a mower not to turn off?
A: No, a bad spark plug typically causes a failure to start or rough running. The shutdown issue is almost always related to the kill switch, wiring, or fuel system.

Q: Is it safe to just pull the spark plug wire everytime?
A: While effective in an emergency, doing this regularly can damage the ignition coil. It’s a temporary fix. You should find and repair the root cause for safe operation.

Q: My riding mower won’t turn off with the key. What do I do?
A: First, use the method in the safety section to stop it. Then, the ignition switch and its wiring are the first places to look. Also, check the alternator; a rare fault there can keep the engine running.

Q: The engine stops only if I choke it. What does that mean?
A: If you have to use the choke to kill the engine, it strongly suggests a problem with the ignition grounding system. The kill wire isn’t working, and you’re stopping it by cutting off air instead. Focus on the switch, lever, and that wire.

Fixing a lawnmower that wont turn off is usually straightforward once you know which system to check. Start with the simple mechanical checks on the control lever and cable. Then move to the electrical connections. With patience and careful safety steps, you can solve this persistent engine trouble and get back to a well-maintained lawn without the worry of a runaway mower. Remember, if you’re ever unsure, there’s no shame in taking it to a repair shop—safety is the most important thing.