How to Fix a Lawn Mower Not Starting: 2026 Guide

A lawn mower not starting is one of the most common seasonal frustrations homeowners face, and in most cases the fix takes under an hour. This 2026 guide covers walk-behind push and self-propelled mowers specifically — if you own a riding mower or zero-turn, component locations and access procedures differ significantly, and ignition or carburetor work should be handled by a certified small engine technician. The four causes below are ranked from most to least likely so you can diagnose the problem efficiently without unnecessary disassembly.

4 Common Causes for Lawn Mower Not Starting

1Stale or Contaminated Fuel

Gasoline begins to degrade in as little as 30 days. Over time, volatile compounds evaporate, leaving behind a thick, varnish-like residue that clogs fuel lines and carburetor passages. Ethanol-blended fuel — which is standard at most US gas stations — also absorbs moisture during storage, causing the water and gasoline to separate into distinct layers that the engine cannot combust. This is the single most common cause of a lawn mower not starting after a period of storage, particularly at the start of spring mowing season.

Symptoms

• Mower cranks normally but will not fire, especially after sitting unused for 30 days or more
• Engine briefly starts on starting fluid but dies immediately when the fluid burns off
• Visible water separation, rust-colored tint, or cloudy appearance in the fuel tank

Care Plan

1. Disconnect the spark plug wire from the spark plug and tuck it safely away from the terminal before doing anything else.
2. Remove the fuel cap. Using a hand pump siphon or a clean turkey baster, draw out as much old fuel from the tank as possible — this is the safest and most universally applicable draining method for all walk-behind mower types. If your mower has a drain plug at the base of the fuel tank, you may use that instead, placing a drain pan beneath it.
3. Wipe out any visible residue from inside the tank with a clean lint-free cloth. Do not reuse any fuel that has been sitting in the tank for more than 30 days, regardless of how it looks or smells. Degradation and moisture contamination from ethanol-blended fuel are not reliably detectable by visual inspection alone. If the mower failed to start, treat all fuel in the tank as suspect.
4. Dispose of the old fuel at a certified household hazardous waste (HHW) facility — never pour it down a drain, onto the ground, or into household trash. To find a certified disposal site near you, visit earth911.com or contact your local municipality’s waste management department.
5. Refill the tank with fresh regular unleaded gasoline (87 octane, maximum 10% ethanol). Add a fuel stabilizer such as STA-BIL or Sea Foam to the fresh fuel according to the product’s instructions — this will protect the fuel for up to 24 months during storage. Reconnect the spark plug wire and attempt to start. If the mower still does not start, the old fuel may have left varnish deposits in the carburetor; proceed to Cause 2.

Common Mistakes

• Assuming visually clear fuel is safe to reuse. Early-stage oxidation and ethanol phase separation are not always visible — if the mower failed to start after sitting, replace all fuel unconditionally.
• Adding fuel stabilizer to old or degraded gasoline. Stabilizer preserves fresh fuel; it cannot restore fuel that has already broken down. Always start with fresh gasoline.

2Clogged Carburetor

Even a small amount of degraded fuel left in the carburetor bowl over winter is enough to leave a sticky, varnish-like residue that blocks the tiny jets and passages the engine relies on for the correct air-fuel mixture. The carburetor is the most mechanically involved repair on this list, but it is well within reach for a homeowner with basic mechanical confidence and about 45 minutes.

Symptoms

• Engine starts for 2–5 seconds on starting fluid but dies immediately without it
• Engine sputters, surges, or runs roughly before stalling
• Visible brown or black varnish residue in the carburetor bowl when removed

Care Plan

1. Disconnect the spark plug wire and drain any remaining fuel from the tank using the method described in Cause 1 before beginning carburetor work.
2. Locate the carburetor (it sits between the air filter and the engine intake, directly beneath or beside the air filter housing). Remove the air filter cover and air filter to access it.
3. Place a small drain pan or folded shop rag beneath the carburetor. Using a flat-head screwdriver, remove the float bowl bolt at the base of the carburetor. The bowl will drop — catch it and the small amount of residual fuel it contains.
4. Locate the O-ring that seals the float bowl to the carburetor body (it sits in a groove around the bowl opening or on the bowl bolt). Remove this O-ring and set it aside before spraying any cleaner — carburetor cleaner solvent degrades rubber quickly, and soaking the O-ring will cause it to swell and seal poorly or crack. With the O-ring removed and set safely aside, spray carburetor cleaner through every jet hole and passage in the carburetor body. Hold the carburetor up to light — you should be able to see daylight through the main jet orifice when it is clear. If the passage is still blocked, soak the carburetor body in a small cup of cleaner for 10–15 minutes and try again.
5. Reassemble the bowl with the original O-ring (or a new one if the original is cracked or deformed — replacements are available for under $2 at most hardware stores). Reinstall the carburetor, air filter, and cover. Reconnect the spark plug wire and attempt to start with fresh fuel. If the mower still will not start after a thorough cleaning, the carburetor may need to be replaced (aftermarket replacement carburetors for common residential engines typically cost $15–$50 and are widely available online). If you are uncertain about any part of this procedure, a small engine shop can clean or replace the carburetor for $80–$150 in labor, with prices varying by region.

Common Mistakes

• Spraying carburetor cleaner onto the bowl O-ring or any rubber gasket. Always remove and set aside rubber components before applying solvent — failure to do so will cause leaks and require a new O-ring.
• Reassembling the carburetor without confirming the main jet orifice is clear. If you cannot see light through it, cleaning is incomplete and the mower will continue to run poorly or not at all.

3Fouled or Failed Spark Plug

The spark plug ignites the air-fuel mixture inside the cylinder. Over time, carbon deposits, oil fouling, a widened electrode gap, or a cracked ceramic insulator can prevent the plug from producing a reliable spark. Spark plug replacement is one of the quickest and lowest-cost repairs in small engine maintenance, and it should be part of every annual service.

Symptoms

• Engine cranks over normally but will not fire at all
• Visible black carbon buildup, oil fouling, or a cracked white ceramic insulator on the removed plug
• Electrode gap is visibly wider than specified, or the electrode is worn down

Care Plan

1. Disconnect the spark plug wire. Use a spark plug socket (typically 5/8-inch or 13/16-inch for most residential mowers) to remove the spark plug.
2. Inspect the plug. If the tip is black (carbon fouled), wet with fuel (flooded), or the white porcelain is cracked, the plug must be replaced. A healthy plug should have a light tan or gray color.
3. Check the gap. Use a spark plug gap tool to ensure the distance between the center and side electrodes matches your engine’s manual (typically 0.030 inches for most Briggs & Stratton and Kohler engines).
4. Install the new plug. Thread the new plug ($5–$10) in by hand first to avoid cross-threading. Once finger-tight, use the socket wrench to tighten it an additional 1/8 to 1/4 turn to compress the washer and create a seal. Reattach the spark plug wire and test.

Common Mistakes

• Overtightening the plug. Aluminum engine heads are soft; using too much force can strip the threads, leading to a very expensive cylinder head repair.
• Assuming a “new” plug is gapped correctly. Plugs can be bumped during shipping. Always verify the gap with a gauge before installation.

4Faulty Ignition Coil

Faulty Ignition Coil (Armature)

The ignition coil is the component that sends high-voltage electricity to the spark plug. While less common than fuel issues, coils can fail due to heat, age, or a buildup of rust on the flywheel magnets. If you have fresh fuel and a brand-new spark plug but still have no ignition, the coil is the likely culprit.

Symptoms

• The mower has no spark when tested with an inline spark tester or by grounding the plug against the engine block.
• The mower starts when cold but dies after 10–15 minutes of use and won’t restart until it cools down (a classic sign of a heat-sensitive coil failure).

Care Plan

1. Access the Coil: Remove the starter recoil housing and the plastic blower shroud from the top of the engine.
2. Inspect the “Kill Wire”: Locate the small wire running from the coil to the handle’s safety lever. If this wire is pinched or the insulation is rubbed through, it will ground out the coil and prevent starting. Repair any frayed wires with electrical tape.
3. Check the Air Gap: The coil must sit a specific distance from the spinning flywheel magnets (typically 0.010 to 0.014 inches). If the bolts have loosened, the gap may be too wide. You can use a thick business card as a makeshift gauge: loosen the coil bolts, place the card between the coil and flywheel magnets, let the magnets pull the coil tight against the card, and retighten the bolts.
4. Replace the Coil: If the wiring and gap are correct but there is still no spark, replace the coil ($20–$80). Most are held in by two 1/4-inch or 5/16-inch bolts.
5. Test for Spark: Before reassembling the shroud, use an inline spark tester to confirm a strong blue spark is present when the starter rope is pulled.

Common Mistakes

• Replacing the coil without checking the kill wire. Many “broken” coils are actually just a $0.05 wire shorting against the metal engine frame.
• Failing to clean the flywheel. If the magnets on the flywheel are covered in heavy rust, the coil cannot “read” them. Clean the magnets with a bit of sandpaper before installing a new coil.