How do I fix a lawn mower that won't start?
Disconnect the spark plug wire first, then check the easy things in order: fuel in the tank and whether it is old, the air filter, and the spark plug. Old fuel is the usual culprit in a mower that sat for weeks. Replace the plug and air filter if they are dirty, and refill with fresh gasoline. If it still will not start, take it to a shop.
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By Kasra Taheri, Founder, LoclisPublished
Researched and drafted with AI, then checked claim by claim against the sources listed below before it was published. Prices and rules change; the dates beside each source say when it was last checked.
Step by step
Read when to stop and call a pro, further down this page, before you start.
Step 1: Disconnect the spark plug wire
Pull the wire off the spark plug before you troubleshoot, so the blade cannot turn if the engine fires.[1]
Step 2: Check the basics
Confirm there is gasoline in the tank, the engine has oil, and any safety bar or handle is held down as the manual describes.
Step 3: Look at the fuel
If the mower sat for weeks, assume the fuel is old; drain it into an approved container and refill with fresh gasoline.[2]
Step 4: Check the air filter
Take the filter out and look at it; a filter full of dust and debris needs replacing as the manufacturer directs.[3]
Step 5: Inspect the spark plug
Remove the plug and look for dirty or scored contacts; fit the plug type and gap the owner's manual lists.[3]
Step 6: Reconnect and try again
Refit the plug and wire, set the mower on level ground, and try a start with the blade disengaged.
Step 7: Stop if it still fails
If it still will not start, stop and take it to a small engine shop rather than dismantling the carburetor.
Start with fuel, filter and plug
A mower that will not start is, most often, short of one of three things: good fuel, clean air or a working spark. Check them in that order, and start by disconnecting the spark plug wire. Virginia Cooperative Extension's rule is plain: "Disconnect the spark or electric plug before troubleshooting or repairing the mower."[1] With the wire off, the blade cannot turn if the engine fires.
Old fuel is the usual suspect
If the mower sat for weeks, suspect the gasoline first. Engines that are rarely or seasonally used, such as lawnmowers or chainsaws, are especially prone to difficulties with ethanol-blended fuel.[2] The fix is fresh gasoline. Drain the tank into an approved container, because you should store gasoline in an approved container with proper labeling.[1] Never refuel the mower when its engine is hot or running.[1]
Penn State Extension suggests that if you think ethanol is the cause, you can track down ethanol-free gasoline and try a tankful to see if it makes a difference.[2] To avoid the problem next time, add fuel stabilizer if the equipment will be idle for more than a few weeks, and avoid keeping stored fuel for long periods of time.[2]
Also check the simple things people forget: oil level, a full tank, and any safety bar or handle that has to be held down while you pull the cord.
The air filter
Air filters keep dust from entering the engine.[3] Over time the filter will become filled with dust and debris,[3] and a choked filter can keep the engine from starting or running well. Take it out and look. If it is clogged, replace it with the part your manual lists. Illinois Extension notes that fuel filters are typically changed on an annual basis,[3] so if the mower has one and it has never been changed, that is cheap to do too.
The spark plug
Pull the plug out with a spark plug socket and look at it. Illinois Extension's advice is to inspect spark plugs, ensuring the contacts are clean and free of scoring.[3] Typically, spark plugs are changed every two years.[3] A black, wet or worn plug is a good reason to fit a new one. Buy the type your manual lists and set the gap it gives, and avoid cross-threading it, then make the final tightening with a spark plug socket wrench.[3]
Try it again
Refit the plug and wire, put the mower on level ground away from where you spilled any fuel, and start it with the blade disengaged. If it coughs but will not run, you are close: the fuel path or carburetor is often the issue.
Stop and call a pro if
- It still will not start with fresh fuel, a clean filter and a good plug. The carburetor, ignition coil or a safety switch may need testing. Do not start taking the carburetor apart for the first time on a Saturday afternoon.
- You smell fuel or see it leaking. Fix the leak before any further starting attempts.
- The pull cord is stuck or the engine is seized. Do not force it.
- The blade is loose, bent or hit something. Disconnect the spark plug wire first and have a shop inspect it. As the Consumer Product Safety Commission puts it, you can disable a mower by choosing to "simply remove the ignition wire from the spark plug".[4]
- You cannot safely tip or hold the mower. Hand it over.
Getting help in Virginia
For more reading, see what a small engine shop charges. Browse small engine repair answers or find local pros.
When you are ready, describe the mower and local small engine shops can quote it.
Common questions
- Why won't my mower start after sitting all winter?
- Usually old fuel. Penn State Extension says engines that are rarely or seasonally used, such as lawnmowers or chainsaws, are especially prone to difficulties with ethanol-blended gasoline. Drain the old fuel into an approved container and refill with fresh gasoline before anything else.
- Is it safe to work on a mower that will not start?
- Yes, if you disconnect the spark plug wire first. Virginia Cooperative Extension says to disconnect the spark or electric plug before troubleshooting or repairing the mower, and never to refuel it when the engine is hot or running.
- How often should I change the spark plug and air filter?
- Illinois Extension says spark plugs are typically changed every two years and the air filter at least once a year, and fuel filters on an annual basis. Your owner's manual gives the exact plug type and gap for your engine.
- Should I use fuel stabilizer?
- Penn State Extension advises adding stabilizer if the equipment will be idle for more than a few weeks, and avoiding keeping stored fuel for long periods. Some small engine makers recommend it at all times, so check your manual.
- When is it time to take the mower to a shop?
- When fresh fuel, a clean air filter and a good spark plug do not get it running, or when you find damage, leaks or a loose blade. A shop can clean or rebuild the carburetor and test the ignition.
Sources
- [1] Lawn Care: Rotary Mower Safety (BSE-42P), Virginia Cooperative Extension, Virginia Tech. Checked October 27, 2026.
- [2] Fuel Ethanol: Hero or Villain?, Penn State Extension. Checked October 27, 2026.
- [3] Mower Maintenance, University of Illinois Extension. Checked October 27, 2026.
- [4] Power Lawn Mowers (walk-behind mower fact sheet), U.S. Consumer Product Safety Commission. Checked October 27, 2026.
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