A lawn mower won’t start when there is bad fuel, a dirty spark plug, a plugged air filter, a clogged fuel system, a dirty carburetor, a clogged fuel cap, or a bad safety switch.
A mower with an electric start may have a bad battery, loose or corroded wires, or a bad starter solenoid.
If you are unsure of your mechanical skills, it’s best to take your lawn mower to a professional repair shop to prevent further damage to the mower or personal injury.
In this guide, I share information on troubleshooting a starting problem, a troubleshooting chart, and a summary of each item that can cause a starting problem.
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Follow all safety instructions provided in your equipment operator’s manual before diagnosing, repairing, or operating. Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.
Troubleshoot a Mower Starting Problem: Fuel, Air, and Spark
An internal combustion engine on a mower requires fuel and air to be introduced to the cylinder. This mixture is compressed and ignited with spark to form combustion.
Before beginning to check for starting problems verify these 4 things:
- The fuel shut-off valve is open allowing fuel to flow out of the fuel tank.
- There is sufficient fresh fuel in the fuel tank.
- The parking brake is engaged or the bail lever is engaged.
- The PTO switch is off (riding mower, lawn tractor, and zero turn).
- The choke is in the closed / on position to start a cold engine.
Check for a Fuel Problem
Old gas is often the root cause for fuel-related issues resulting in components not functioning or clogs developing.
Before checking for a fuel problem, if your mower has a fuel shut-off valve, make sure it is in the open position. This valve is often moved to the closed position when the mower is stored or for transportation.
Perform this test to narrow down your problem to the fuel system:
- Detach the air filter cover and remove the air filter.
- Spray carburetor cleaner into the air intake. Find out why I use carburetor cleaner and not starter fluid.
- Attempt to start your mower using the manual recoil starter or turning the ignition key (depending on your mower type)
- If the mower attempts to start or starts, then you have a problem with the fuel system.
- The most common fuel issues are a dirty carburetor, clogged fuel filter, and clogged fuel lines. The fuel pump may also be an issue if your mower uses one.
- If the mower doesn’t attempt to start, you most likely have a spark plug problem.
- If the mower attempts to start or starts, then you have a problem with the fuel system.
Check for an Airflow Problem
Air is an important component needed for your engine to start. The engine must run rich to start a cold engine. This condition allows more fuel and less air into the cylinder.
Airflow is controlled by the choke. You will find a choke lever that opens the choke plate to introduce more air and close the choke plate to restrict air.
On newer push mowers, you will find an automatic choke controlled by a thermostat.
Perform this test to narrow down your problem to the air supply system:
- Detach the air filter cover and remove the air filter.
- Inspect the air filter’s condition. A clogged air filter will restrict airflow. Clean the filter or replace it if it is very dirty or damaged.
- With the air filter removed, inspect the choke plate.
- Manual-choke model
- Move the choke lever to the on position: The choke plate should be closed.
- Move the choke lever to the off position (on some models, this is placing the throttle in the fast throttle position): The choke plate should be open.
- Auto-choke model
- The choke plate should be closed when the engine is cold and moved to the open position when the engine warms.
- Manual-choke model
- If the choke isn’t opening and closing correction, check the choke linkage and choke cable. Loosen a stuck choke by lubricating the linkages and choke shaft using carburetor cleaner. Replace a bad choke cable (if used on your model).
Check for a Spark Problem
Spark must be released at the right time for combustion. A spark plug may fail to spark if it is dirty or damaged. It may also fail due to a loose spark plug wire, a bad ignition coil (armature), or other ignition system problems.
Perform this test to narrow down your spark problem:
- Remove the spark plug wire (boot) from the spark plug.
- Attach a spark plug tester to the spark plug ignition wire.
- With the spark plug still installed, attach the other end of the spark plug tester to the spark plug.
- Attempt to start the engine using the pull cord or electric start.
- You will see a glow in the transparent section of the tester if it is generating a spark.
- If you don’t see a glow, the spark plug may be bad or you may have a bad ignition coil. On electric start models, you may have a problem with the ignition system including the battery, wiring, and starter solenoid.
Lawn Mower Starting Issues: Push Mower
1. No Fuel
Even though you know a lawn mower requires gas to start, I’m going to mention it anyway. Sometimes when you’re frustrated with your mower you skip over the simple things when diagnosing your problem.
Solution: Fill your gas-powered lawn mower with gasoline that has an octane rating of 87 or higher and an ethanol level no greater than 10%. Read more about choosing the right gas for your lawn mower here.
2. Bad or Old Gas
Gas can begin to break down and become less effective as soon as 30 days after you purchase the fuel. The sticky substance left behind by ethanol and moisture in the fuel can begin clogging your fuel system including the fuel lines, fuel filter, and carburetor.
When you are not able to use your fuel within 30 days, you should add a fuel stabilizer. Add this to the gas you run in your fuel tank in addition to any gas remaining in a storage container.
Solution: Remove the old fuel. Flush the tank and add fresh fuel. Add a fuel additive to clean your fuel system and stabilize it. I have had good results using a product called Sea Foam Motor Treatment. You can read more about why I use Sea Foam here.
3. Bad Fuel Cap
Without a vent in the fuel cap, the tank will act like a vacuum and restrict fuel from flowing through the fuel lines. Old fuel can cause clogging in the vent so air will no longer pass through the cap.
To isolate your cap as being the cause of your lawn mower starting problem, start and run your mower for a while with the cap off and then with it on to see if your cap affects the running ability of your lawn mower.
Solution: You may be able to clean your fuel cap and unclog the vent. If you are unable to remove the clog, replace your cap with a new one.
4. Bad Spark Plug or Loose Connection
A spark plug may no longer work and cause intermittent starting and running issues with your lawn mower. A spark plug that is dirty from carbon buildup must be cleaned or replaced if the tip appears very dark in color.
A damaged spark plug that has a broke porcelain insulator or burnt electrode must be replaced as well. To minimize the problems you have during the mowing season from a spark plug, it is best to start a season with a new plug. Some lawn mower engines require 2 spark plugs.
Starting problems can also be caused by a spark plug that is incorrectly gapped or has loose spark plug connections.
Solution: Remove your spark plug and inspect it for signs of carbon buildup or cracked porcelain insulators. Replace with a new spark plug(s). Make sure to gap them according to manufacturer specifications and that the spark plug wires are securely attached.
5. Clogged Air Filter
A lawn mower requires clean air to run. The air filter prevents dirt and debris from entering the air intake and contaminating the engine. Dirt in the engine can cause significant engine damage.
Never run your mower without an air filter even if it’s only for a short period of time while you source a new filter.
When an air filter gets plugged so air is no longer able to pass through the filter, your mower won’t start. It is important to regularly check the air filter and keep it clean. By checking, cleaning, and replacing this inexpensive part when needed, you can prevent an expensive engine repair.
Solution: Carefully remove the air filter from the air filter housing so you don’t allow dirt to fall into the air intake. If you find dirt in the housing, wipe it out with a clean cloth. Follow one of the following procedures for your type of air filter:
Clean a paper lawn mower air filter
- Knock out the excess dirt in the filter by tapping it against a solid surface.
- Hold the filter up to a light source and check for light shining through the paper element.
- Reuse the filter if you can see light pass through the paper. If you cannot, replace your filter with a new air filter.
- Install the air filter and attach the cover.
Clean a foam lawn mower air filter
- Determine whether you can reuse your filter before cleaning it. If your filter has dark spots or is dry and brittle, replace the filter with a new one.
- If your filter is in good condition, proceed with washing it with mild dish soap and water to remove dirt from the filter.
- Rinse the filter and lay it flat to dry. Placing it outdoors in the sun will speed up the process.
- Once the filter is dry or if you are using a new foam filter, add a foam filter oil to lightly saturate the filter. You don’t want it dripping with oil.
- Install the filter into the housing and attach the cover.
6. Clogged Fuel Filter
The fuel filter’s function is the strain the fuel coming out of the fuel tank to prevent dirt from entering the fuel system and engine. If your fuel filter is clogged and prevents fuel from passing through it, it must be replaced.
Solution: Replace a plugged fuel filter. You may find your mower uses an inline fuel filter placed between the fuel lines. If you don’t see one there, you may have a fuel filter inserted into the bottom of the fuel tank.
7. Clogged Fuel Line
A lawn mower fuel line can become clogged by dirt and the sticky substance left behind by old gasoline. This keeps fuel from getting to your carburetor and to your engine. Read more about identifying a clogged fuel line here.
Solution: Remove the fuel line, spray carburetor cleaner into the tube and use compressed air to blow air through the tube until the line is free of dirt and gummy residue. Repeat as necessary. Replace the fuel line with a new line when you can’t remove the clog.
8. Dirty Carburetor
Your mower uses a carburetor to regulate the amount of gas mixed with air allowed into the cylinder to form combustion.
The additives added to fuel, including ethanol, can cause gummy substances to form in your carburetor. The substance clogs the small parts in your carburetor restricting fuel.
Solution: If you are somewhat mechanical, you can try to clean the carburetor on your lawn mower. If you are not, have a local lawn mower repair shop perform the work.
You may choose to replace the carburetor if it appears to be in very bad condition. Have a small engine repair shop clean the carburetor if you don’t want to attempt the cleaning or rebuilding the carburetor.
9. Bad Battery or Loose Wiring (Electric Start)
Lawn mowers with an electric start require a power source like a battery. Make sure your cables and battery terminals are secure.
Clean any corrosion you find on your terminals using a baking soda solution (2 cups water to 3 heaping tablespoons of baking soda). Once you confirm you have a good connection, continue testing the battery.
Solution: Test your battery with a multimeter. Replace a bad battery.
10. Bad Ignition Switch (Electric Start)
Your lawn may have an ignition switch that has failed. If you turn the key in your ignition switch and nothing happens or it just doesn’t feel right, you need to check your switch using a multimeter.
Solution: Replace the ignition switch if it is bad.
11. Bad Recoil (Manual Start)
Manual start push mowers utilize a recoil to start the mower. The recoil can break and you are no longer able to start your mower.
Solution: If the rope is no longer wrapped around your recoil, you may be able to restring it to get it working again. You may have a broken pulley, spring, or clips that need to be replaced.
If you find broken parts, you should price out the parts in addition to the whole recoil assembly. It may be more cost-effective to replace your recoil.
12. Bad Starter Solenoid (Electric Start)
A lawn mower solenoid on your lawn mower is an electromagnetic switch that is like an on-off switch that actuates the starter motor to turn over the engine.
A click or hum when turning your ignition key is an indication to check your solenoid. Another indication your lawn mower solenoid may be bad is when a wire attached to your solenoid gets hot and begins to smoke or melt.
Solution: Test your lawn mower solenoid by following the steps here. Replace your solenoid if it is found to be bad.
13. Bad Safety Switch
The safety switch is designed to kill the engine when the operator is no longer present. For example, if you let go of the bail lever, the engine stops.
A faulty switch may not recognize when the bail lever is engaged to start the mower. Refer to your operator’s manual for the other types of safety switches used in your type of lawn mower.
Solution: You can temporarily bypass the safety switch to identify a bad switch. Do not operate a mower without the safety switch installed for your safety. Always have safety switches installed and working on your equipment. Replace a bad switch.
14. Incorrect Starting and Operating Procedure
Your lawn mower has safety features that won’t allow your mower to start unless you follow its starting procedures. This may be engaging the bail lever and adjusting the choke. Starting procedures vary depending on the type of lawn mower you are using.
Solution: Refer to your lawn mower’s operating manual to ensure you are operating your lawn mower correctly, so you don’t set off the safety features that shut off your lawn mower or don’t allow it to start.
Mower Starting Problems Causes & Solutions
|Cause||Reason for Problem||Solution|
|No gas in the fuel tank||Fuel tank is empty||Fill with fresh fuel|
|Bad or old fuel||Fuel breaks down overtime making your fuel less efficient and prone to clogging the fuel system||Drain the fuel tank and fill it with fresh fuel. Use a fuel additive like Sea Foam to stabilize the fuel and assist with cleaning the fuel system. Use the right fuel.|
|Faulty or clogged fuel cap||The vent in the cap can get clogged causing your tank to form a vacuum restricting fuel flow||Use a thin piece of wire to try to clear the clogged vent. Replace the cap if needed.|
|Bad spark plug||Excessively dirty or broken spark plug; bad connection; not gapped correctly||Replace with a new spark plug, secure connections, and ensure it is gapped to manufacturer specifications|
|Plugged air filter||Dirt and grass can plug the air filter preventing airflow||Remove the filter and clean it. Replace the filter if it is in bad condition.|
|Plugged fuel filter||A dirty fuel filter will restrict fuel flow||Replace the fuel filter|
|Blocked fuel lines||Bad fuel deposits and dirt can build up and clog the fuel lines preventing the engine from getting the required fuel||Use a carb cleaner and compressed air to clear the clog out of the fuel line. Replace the line if necessary.|
|Dirty carburetor||The carburetor can become dirty and clogged due to bad fuel and ethanol deposits preventing fuel flow||Clean the components making up the carburetor. Replace if needed.|
|Bad battery or loose & dirty terminals (electric start)||A dead battery, loose cables, or corroded terminals can cause starting problems||Charge your battery and replace it if it won’t hold a charge. Make sure your cables are tight and clean the corrosion from terminals.|
|Bad safety switch||Safety switches are a safety measure that exists to prevent your mower from starting in certain situations. When a switch fails, your mower may not start.||Test and replace faulty safety switch|
|Bad ignition switch (electric start)||Your mower will not start when the ignition switch is faulty.||Test and replace a bad switch.|
|Broken recoil (manual start)||A pull start mower may have a damaged or broken recoil preventing the recoil to initiate starting your engine||Check the recoil to see if it can be restrung or if broken parts, like the pulley, can be replaced. Replace the recoil assembly if needed.|
|Bad starter solenoid (electric start)||An ignition switch that hums when turning the key indicates you may have a problem with the starter solenoid causing a starting problem.||Test the starter solenoid and replace it if bad.|
Still Having Problems with Your Lawn Mower?
Lawn mower ownership doesn’t come without its frustrations. Own a mower long enough, you are bound to run into many lawn mower problems including starting, smoking, leaking, cutting, and overheating.
For mower troubleshooting, check out my guide Common Lawn Mower Problems: Solved.