If you have a four-stroke mower, it's worth checking the spark plug. If you have a two-stroke, replacing it can give your mower a new lease of life. Use the socket set to remove the spark plug. If it doesn't need replacing, clean it with the wire brush. When replacing the spark plug, screw it in with your fingers as far as you can, so that you don't cross-thread it. Then tighten it with the socket. Be careful not to overtighten it.