Lawn Mower Won’t Start After Sitting All Winter: 7 Fixes That Work

By Graham Fisher

Updated November 2026

A lawn mower that won’t start after sitting all winter is usually suffering from one of a handful of predictable problems: stale fuel, a fouled spark plug, a dead or weak battery, or a carburetor gummed up by old gasoline. This guide walks through the checks in the order that saves you the most time, so you can rule out the simple causes first and only reach for tools when you need them. Most owners can get their mower running again in an afternoon.

Why Mowers Fail to Start After Winter

Gasoline is the main culprit. Modern gasoline contains ethanol, which attracts moisture and begins breaking down within a few weeks. Over a winter of sitting, it can leave varnish-like deposits in the fuel system, clog the tiny jets in the carburetor, and prevent the engine from drawing a proper mixture. Oil can also drain back into the cylinder, and the spark plug may corrode or foul. On electric or battery-equipped mowers, the battery is the usual weak point, since cold storage and self-discharge can leave it too low to crank the motor.

Shopping for one? See our guide: Lawn Mower Won't Start After Running Out of Gas: 3 Best Starting Aids Compared.

Before you start working, make sure the mower is on a level surface, the blade is disconnected from any power source, and the engine is cool. If you are unsure about any step involving the fuel system or electrical components, consult a qualified small-engine technician.

Step-by-Step Fixes for a Mower That Won’t Start

Step 1: Check the Fuel

Look at the fuel in the tank. If it is dark, smells sour or varnished, or has been sitting since last fall, drain it. Old fuel is the single most common reason a mower refuses to start in spring. Dispose of it at a local hazardous waste facility, not down a drain. Refill with fresh gasoline, ideally with no more than 10 percent ethanol (E10 is the most common blend in the US). If your mower is designed for ethanol-free fuel, use that instead.

Step 2: Inspect the Oil Level and Condition

Check the dipstick. If the oil looks milky, dark, or is above the full mark, it may have picked up fuel or moisture. Change the oil according to the owner’s manual. Many manufacturers specify roughly 0.6 to 0.8 quarts for common push mower engines, but confirm the exact capacity for your model.

Step 3: Examine and Replace the Spark Plug

Remove the spark plug with a socket wrench. Look at the electrode. A black, sooty tip suggests a rich fuel mixture, while a white or blistered tip suggests overheating. Clean any buildup with a wire brush, then check the gap against the manufacturer’s specification, which is commonly around 0.030 inch for small engines. If the plug is worn or heavily fouled, replace it with the correct type listed in the manual. A new plug costs little and eliminates one common variable.

Step 4: Confirm the Spark

Reinstall the plug, keeping the wire attached. Ground the threaded part of the plug against the engine block, then pull the starter cord or crank the engine. A blue, consistent spark means ignition is working. No spark or a weak orange spark points to a bad plug, a faulty ignition coil, or a problem with the kill switch or wiring.

Step 5: Test the Battery (Electric and Riding Models)

For battery-powered or riding mowers, measure the battery voltage with a multimeter. A fully charged 12-volt lead-acid battery typically reads around 12.6 volts or slightly higher. Readings below about 12.2 volts suggest a discharged battery. Charge it fully using a charger matched to its chemistry. If the battery will not hold a charge after several cycles, replace it. Clean corroded terminals with a baking soda and water solution, then rinse and dry them before reconnecting.

Step 6: Clean or Rebuild the Carburetor

If the fuel is fresh and the spark is good but the engine still won’t start or sputters and dies, the carburetor is the next suspect. Shut off the fuel valve, remove the carburetor bowl, and inspect the jets and passages. Spray them with carburetor cleaner and use a fine wire or compressed air to clear blockages. Do not poke metal wire into the calibrated jets, as you can enlarge them. Replace the bowl gasket and float if they are cracked or stiff. A carburetor rebuild kit contains the gaskets, diaphragms, and needles needed for most small engines.

Step 7: Check the Air Filter and Choke

A clogged air filter starves the engine of air, causing hard starting and poor running. Remove the filter and inspect it. Foam filters can be washed in mild soap and water, then oiled lightly. Paper filters should be replaced if they are dirty or oily. Also check the choke: on a cold start, it should be closed, then opened once the engine fires. A choke that sticks can flood the engine.

Quick Checklist Before You Pull the Cord

1. Replace or drain old fuel and add fresh gasoline.
2. Check oil level and change it if it looks contaminated.
3. Inspect, clean, or replace the spark plug and confirm the gap.
4. Verify a strong spark at the plug.
5. Test and charge the battery on electric or riding models.
6. Clean the carburetor bowl, jets, and passages.
7. Replace or clean the air filter and confirm the choke operates correctly.

Tools and Supplies You May Need

Common Mistakes to Avoid

• Adding more fuel to a flooded engine. Remove the spark plug, pull the cord several times to clear excess fuel, then reinstall and try again.
• Using the wrong spark plug. Mismatched heat range or thread size can cause misfires or engine damage.
• Overtightening the spark plug. Snug it, then give it a small additional turn if the manual calls for a specific torque.
• Ignoring the fuel line. Cracked or hardened fuel lines can leak air and prevent proper flow.
• Skipping the battery on electric models. A weak battery can mimic a motor or wiring fault.

Tips for Next Winter

Prevention is far easier than repair. Before storing the mower, run the engine until it stops on its own so the carburetor empties, or add a fuel stabilizer to full fuel and run it for a few minutes. Change the oil, replace the spark plug if it is worn, and store the mower in a dry, sheltered place. Disconnect or charge the battery on electric models, and keep it on a maintainer over the off-season.

Recommended gear

• Fuel stabilizer: Keeps gasoline from breaking down during storage, reducing the varnish that causes most no-start problems in spring. View on Amazon
• Carburetor cleaner spray: Dissolves varnish and gum in jets and passages without disassembling the entire carburetor. View on Amazon
• Digital multimeter: Lets you check battery voltage and test circuits so you can narrow down electrical faults quickly. View on Amazon
• Feeler gauge set: Measures spark plug gap accurately to the manufacturer’s specification. View on Amazon

If you are working through a specific brand or model, related guides may help narrow down the cause. For example, Lawn Mower Won't Start After Sitting: 8 Causes and Fixes covers additional causes beyond winter storage, and Push Lawn Mower Won't Start After Winter: 7 Fixes to Try First focuses on walk-behind models. If your mower has a carburetor issue after a repair, see Lawn Mower Won't Start After Replacing Carburetor: 6 Parts Compared. For a riding model, Riding Lawn Mower Won't Start After Winter: 7 Fixes to Try First addresses the battery and safety switch checks specific to those machines.

FAQ

How long can gas sit in a mower before it goes bad?

Gasoline with ethanol can start to degrade within about 30 days, and after several months it often causes starting problems. Using a fuel stabilizer or running the engine dry before storage extends the usable life considerably.

Can I use starting fluid on a mower that won’t start?

Starting fluid can help confirm whether the problem is fuel or ignition, but it should be used sparingly. Overuse can damage components and is not a fix for old fuel or a dirty carburetor.

How do I know if the spark plug is bad?

A plug with heavy carbon, oil fouling, cracked porcelain, or an eroded electrode should be replaced. If the spark is weak or absent when grounded, replace the plug and check the ignition coil if the problem continues.

Why does my mower start and then die?

This usually points to a clogged carburetor jet, a partially blocked fuel line, a dirty air filter, or a vent in the fuel cap that is stuck closed.

Should I charge the battery before trying to start the mower?

Yes. A low battery can cause slow cranking or no response, and it is one of the easiest things to check on electric and riding mowers. Charge it fully and inspect the terminals for corrosion first.

Is it safe to drain gasoline myself?

Draining fuel is generally manageable with proper ventilation and no ignition sources nearby. Dispose of it at a hazardous waste site. If you are uncomfortable handling fuel, a small-engine shop can do this for you.

When should I take the mower to a repair shop?

If you have replaced the plug, provided fresh fuel, cleaned the carburetor, and charged the battery without success, or if you notice fuel leaks, smoke, or unusual noises, consult a qualified small-engine technician.

Will a lawn mower that sat all winter need a tune-up?

Most mowers benefit from an annual tune-up that includes an oil change, spark plug replacement, air filter service, and blade sharpening. Following the manufacturer’s maintenance schedule keeps the engine reliable season after season.

Conclusion

Most lawn mowers that won’t start after sitting all winter can be revived with fresh fuel, a new or cleaned spark plug, a charged battery, and a clean carburetor. Work through the checks in order, starting with the fuel, because that is where most problems begin. A few minutes of storage prep each fall will prevent most of these problems the following spring.

Related reading

• Toro Lawn Mower Won't Start After Winter: 7 Fixes That Work
• Toro Lawn Mower Won't Start After Sitting: 7 Causes and Fixes

About Graham Fisher

Graham Fisher is part of our editorial team and reviews and edits our lawn mowers guides, checking product details against manufacturer specifications and owner feedback.