My 21" self-propelled mower won't start. The lights and self-propel work, but the engine won't come on. All summer it's been running for a few minutes, then cuts off. Sometimes the battery or mower flashes different color lights or a solid light. I wait a few minutes, then run a few minutes etc...But now, it won't come on at all. Battery appears just fine. I don't have a large area to mow and I never let my grass get too long. Worked just fine all of last year, it's about 1 1/2 years old.