Can Old Gas Stop a Lawn Mower From Starting?

The answer

Yes. Old or stale gasoline can contribute to a lawn mower not starting or running properly.

Why

Briggs & Stratton says old gas can lead to starting problems and notes that stale untreated gasoline begins to break down after about a month. Stale fuel can also leave deposits that affect the carburetor and fuel system.

When it is usually OK

• The mower sat for an extended period with untreated gasoline.
• Starting problems began after seasonal storage.
• The fuel is old enough that freshness is questionable.

When you should not

• Fresh fuel is already in the mower and the problem remains.
• The mower has another independently confirmed fault.
• The engine manual specifies a different fuel-related diagnosis.

Risks and exceptions

• Stale fuel can make starting unreliable.
• Untreated stale fuel can contribute to carburetor deposits.
• Replacing fuel alone will not fix unrelated ignition or air-filter problems.

Best practice

If fuel has been stored long enough to be stale, follow Briggs & Stratton guidance for removing old fuel and replacing it with the correct fresh fuel before more invasive diagnosis.

Alternatives

• Check the spark plug if fresh fuel does not solve the issue.
• Check the air filter.
• Use authorized service if basic checks fail.

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