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6 Reasons Your Lawn Mower Won’t Start After Winter

You stored your mower at the end of the mowing season in working condition. However, the mower won’t start after the winter season. Storing a mower with untreated fuel or in very cold or wet conditions can contribute to starting problems.

A lawn mower won’t start after winter when the gas is old causing fuel restrictions in the fuel line and carburetor; the battery is weak or bad; the spark plug is corroded or dirty, or the electrical wiring and components are corroded or damaged.

Follow all safety precautions listed in the operator’s manual when working on the mower. This includes removing the spark plug wire(s) before beginning repairs.

Lawn mower won't start after winter

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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.

How to Troubleshoot & Start a Lawn Mower After Winter

  • Make sure the fuel shut-off valve is open: You may have shut off the valve before storing the mower. Open the valve so fuel is able to flow.
  • Check for a weak or bad battery: Check the battery’s voltage and charge it if the reading is low. If your battery isn’t holding a charge, replace it with a new one.
  • Check for dirty or corroded spark plug: Remove and inspect the spark plug. Replace it if you find signs of corrosion or if it is very dirty.
  • Check for bad fuel: If fuel has been sitting in the mower after winter without a good fuel stabilizer, it must be replaced with fresh fuel. Additionally, replace the fuel if you find water in the tank.
  • Check for sufficient fuel flow: Remove the air filter and spray carburetor cleaner into the air intake and start the mower. If it runs for a bit and then shuts down, you have a fuel restriction preventing the engine from getting enough fuel to start and run. Proceed with checking for a restriction in the fuel lines, fuel pump (if the mower uses one), and carburetor.

Reasons a Lawn Mower Won’t Start After Winter

Old Gas

Gas is often the culprit of a mower that’s been sitting around all winter with old gas. Fuel begins to break down rather quickly. In fact, it can begin degrading as soon as 30 days after purchase.

Most gas contains ethanol. Ethanol is an alternative fuel added to gas to make it a little environmentally friendly. While ethanol is better for the environment, it is not good for the small engine in your lawn mower.

While gas is required to keep your lawn mower running, it can also be the root cause of your starting problem after winter. Ethanol attracts moisture to the fuel system.

This ethanol and water mixture will separate from the gas and sink to the bottom of the fuel tank. Water is not combustible. This mixture can also leave behind varnish and gummy deposits restricting fuel flow and damaging fuel components.

When the engine isn’t able to get sufficient fuel or the fuel contains water, the mower won’t start.

When you find old fuel in your riding mower, drain the fuel tank and add fresh fuel. Add a fuel additive like Sea Foam Motor Treatment to your gas to clean and remove moisture from the fuel system.

Read more about making the right fuel choice and caring for your lawn mower fuel here.

Clogged Fuel Lines

The sticky substance left behind by old fuel can stick to the fuel line keeping a good flow of fuel from passing through the line. You’ll need to check the fuel line to ensure it is allowing sufficient fuel flow to the fuel pump and carburetor.

Locate a clogged line by stopping the fuel flow using the fuel shut-off valve or fuel pinch-off pliers. Remove the end of the line from the fuel pump inlet port (or carburetor if your mower doesn’t have a fuel pump) and place it in a container to collect fuel.

Make sure the container is placed lower than the fuel tank because fuel cannot run uphill without the assistance of a fuel pump.

Start the fuel flow and check the flow coming out of the line into the container. If you have good flow, reinstall the fuel line. If you don’t get a good flow, shut the fuel flow off and remove that section of line from the lawn mower.

Spray carburetor cleaner into the line. What you’re trying to do is loosen up the clog. Next, blow compressed air into the line to free the line of the clog.

Repeat spraying with carburetor cleaner and blowing the line out with air until the blockage is removed.

Reinstall the line once it is free of the clog. Install a new section of the fuel line that is the same length and diameter when you are unable to unclog the line or the line is cracked and prone to leaking.

Dirty Carburetor

A little fuel is stored in the carburetor fuel bowl after it leaves the fuel tank. Old fuel can leave sticky and crusty deposits in the carburetor.

The fuel jet can become clogged and the small components can become stuck not allowing the proper amount of fuel to get to the engine to form combustion.

If you’re getting sufficient fuel flow to the carburetor, but fuel isn’t getting to the engine, the mower won’t start. You will have to take your carburetor apart and clean it with carburetor cleaner to remove the buildup preventing it from functioning properly.

Follow the instructions to clean the carburetor here. If you are unable to sufficiently clean it or find any damaged parts, you may have to rebuild it or replace it with a new carburetor.

Bad Spark Plug

When your lawn mower has been sitting over winter or another long period of time, the spark plug can begin to corrode causing starting and running problems.

Remove the spark plug with a socket wrench and inspect it for signs of corrosion. Replace a spark plug that is corroded, very dark in color, or damaged.

If your spark plug is in good condition, but dirty or covered with carbon, clean it with a wire brush. Install the new or cleaned spark plug after you confirm it is gapped to the engine manufacturer’s specification. Then, securely attach the spark plug wire once all repairs have been made.

Bad or Dead Battery Keeps

If your lawn mower uses a battery to start it, you need to check it. A lawn mower battery left in the cold can freeze and no longer hold a charge. You don’t have to worry about your battery freezing if it is fully charged.

However, if it is not, there is a good chance you ruined your battery over the cold winter season. See the chart below for the temperatures when batteries freeze when they are not fully charged.

Battery charge12-volt batteries6-volt batteriesBattery Freezing Temperature
100%12.78 V6.39 V-80 degrees Fahrenheit
92%12.66 V6.33 V-71.3 degrees Fahrenheit
85%12.57 V6.28 V-62 degrees Fahrenheit
62%12.27 V6.13 V-16 degrees Fahrenheit
50%-10 degrees Fahrenheit
40%11.97 V5.98 V+5 degrees Fahrenheit
20%11.67 V5.84 V+19 degrees Fahrenheit
Zero charge+20 degrees Fahrenheit
Winter Battery Freezing Temperatures

Check the battery to determine its charging level using an ohmmeter. Most lawn mowers use a 12-volt battery. A fully charged battery is one that reads 12.7 volts.

If it is less than this, charge the battery using the steps below. If you find your battery won’t hold a charge, replace it with a new lawn mower battery.

Read more about properly caring for and storing a battery in A Guide to Winter Lawn Mower Battery Care.

Charging a lawn mower battery

Charging a lawn mower battery requires a battery charger and cables. You should always wear protective gear before attempting to charge your lawn mower battery. Here are the steps for charging a lawn mower battery:

  • Put on your safety gear so that your eyes and skin are protected from acid or electrical shock.
  • Get access to the battery and its terminals. You may need the screwdriver to uncover the lawn mower’s body to get access to the battery or battery casing.
  • Leave the battery in its casing with the terminal cables attached.
  • Connect the charging cables starting with the positive cable first (The red one with the plus sign on it)
  • Connect the negative cable next (The black one with the negative sign on it). Remember: Positive cable (Red +) is the last one off and the first one on.
  • Make sure that your skin only touches the rubber coating of the charging cables and clamps.
  • Set the charger’s voltage level and amp level to the desired level. The average volt level for lawn mowers is usually 12 volts. More amperage charges the battery faster (Start with two amps and work up to no more than 10 amps).
  • If your charger has a battery charging gauge, keep the charger connected until the battery is fully charged. A 10 amp charger usually takes about one hour for full charging of your lawn mower battery.

Corroded Electrical Components

Storing your lawn mower when it isn’t free of debris or moisture can cause corrosion to develop on your components. Check your safety switches and wiring to make sure they make good connections and are not corroded.

Rodent damage can also cause electrical component damage. I have seen rodent nests built in lawn mowers sitting over winter. Lawn mowers can become homes for rodents and the wires are often chewed and damaged.

When you find rodent damage, the damaged wiring and parts must be replaced. Use a rodent deterrent like Grandpa Gus the next time you store your mower. This product is great because it comes in packets to place around the mower and a spray for the wiring.

Common Winter Storage Questions & Answers

Should You Remove Your Lawn Mower Battery for Winter Storage?

A battery must either be removed from your lawn mower or fully charged when leaving it in your mower for winter storage.

A fully charged battery will not freeze. If you are not confident your battery is fully charged or you just want peace of mind it won’t freeze, remove the battery. Store it in a cool dry location away from any moist or humid areas.

Should You Empty the Lawn Mower Fuel Tank for Winter Storage?

It’s your choice whether or not you drain the fuel tank before storing your mower over winter. You can either drain the fuel tank or leave fuel in the tank. However, if you choose the latter, you must add a fuel additive to stabilize the gas.

Not all fuel additives are created equal, so make sure you are adding a good quality additive that keeps the gas stable for a long period. I like Sea Foam Motor Treatment or STA-BIL Storage fuel additive.

Note: This stabilizer only works with fresh gas. It cannot reverse the effects of old gas. Read more about why I choose to use Seas Foam in my lawn mower in Use Sea Foam in a Lawn Mower to Stabilize Your Fuel.

Follow these instructions for the method you choose to use when preparing your lawn mower for storage during the winter season:

Empty the lawn mower fuel tank for winter storage

Use a fuel siphon pump and empty the fuel tank into a container designed to hold fuel. Start and allow your lawn mower to run until it stops because the engine is no longer getting fuel. This will allow most of the fuel to be removed from the fuel system.

Stabilize your gas when leaving fuel in the fuel tank for winter storage

Add a fuel stabilizer to fresh gasoline, as instructed on the bottle, to mix it. Add it to the fuel tank.

Once you have the fuel and stabilizer mixed and added to the tank, start and allow the mower to run for a few minutes so the treated fuel works its way through the system before storing.

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.