Lawn mower engine troubles tend to appear after the mower is stored for the winter and brought out for trimming in the spring. Always winterize your mower by removing all liquids from inside the engine before storing it away. Troubleshoot your engine by narrowing down where the engine failure is originating.
Check the engine for the proper amounts of oil and gasoline. Most modern mowers have a fill line that indicates when the liquids are at the right level, or a dipstick to check the level on. Replace your fuel if it is more than a few months old or if there is water present.
Inspect the spark plug by removing it from the machine with a spark plug ratchet. Examine the end of the plug. If it is black or rusty in color tap the end against a piece of metal. A blue spark should fly up. Replace the spark plug if it does not make a spark or if the spark is a different color.
Check the cords leading from the safety device of the mower, such as a safety handle. Tighten any loose connections. A mower will not start if the safety handle is not engaging properly. Replace any lines that are broken or damaged in any way.
Remove the cover of the air filter and inspect it for signs of dirt and debris. Replace the filter if it is made of paper. Rinse foam filters using water and liquid detergent.
Check the connections to the battery (if the mower has one) and tighten with a screwdriver if necessary. Clean any terminals using a wire brush if there is debris that may reduce the strength of the connection.