According to Toro, the three most common reasons that a Toro mower won't start is that the fuel is stale (older than 30 days), the spark plug needs replacing, or the air filter is dirty and is not drawing in enough air.
On electric-start mowers, the battery may need charging. Batteries should be charged at the beginning and end of the season.
Consumers add that a common cause of non-starting Toro mowers is that the carburetor is clogged, often with overwintered gas. The carburetor should be flushed and perhaps rebuilt by a service professional.
Fuel should be left in the tank no longer than 30 days. This is because the most volatile elements in fuel, the ones that are most helpful in starting the engine, deteriorate most quickly.
Either put no more than a 30-day supply of fuel in the tank, or add a fuel stabilizer to the tank when adding fuel, which can help gas last up to 6 months.
If you have old fuel in your tank, drain the old fuel and flush the system, then add fresh fuel.
Replace Spark Plug
Spark plugs ignite the fuel, and if they are dirty or no longer working should be replaced. Fouled spark plugs will have black carbon deposits on the tip.
They are inexpensive, so if you are in doubt, just replace the spark plug because a new one will work much more effectively than an older one.
Dirty Air Filter
The air filter filters the air, which is sucked into the mower's engine and allows the fuel to be ignited by the spark. A dirty air filter will not allow enough air into the engine.
To check if your air filter is dirty, remove the air filter casing and inspect the filter. A clean filter will be white; a dirty filter will have black or gray deposits.
Toro recommends replacing your air filter annually.
Electric-start mowers have a battery in the starter, which needs to be charged an average of twice a year, at the beginning and end of the season.
Toro consumers report that a common cause of non-starting Toro mowers is a clogged carburetor. Possible causes are debris in the fuel, or fuel left for many months in the fuel tank. Fuel that is left in the fuel tank for many months can coagulate and clog the carburetor.
If you suspect the carburetor is clogged, the only way to check and to fix it is to remove the carburetor, disassemble, clean, then reassemble. This should be done by a qualified service professional.
To prevent this issue, add a fuel stabilizer then run the mower briefly to distribute the stabilized fuel through the system, before storing your mower for the winter.