As the mowing season winds down and temperatures start to plunge, packing away your lawnmower is usually the final task on the fall yard work checklist. However, simply wheeling your mower into the shed or garage with a half-full tank of gasoline is a costly mistake.

Stagnant, untreated gas is the single biggest cause of small-engine failure, leading to gummed-up carburetors, rusted fuel lines, and an expensive repair bill — or a completely dead engine when spring rolls around.

Before you store your equipment for the cold months ahead, here is why untreated fuel ruins your mower during seasonal storage, and the simple one-step solution that keeps your engine protected.

1. Why gas goes bad

Most gasoline purchased at the pump contains up to 10 percent ethanol, which works fine when you are burning through a tank every week.…