If the toilet is coming on, and shutting back off it is most likely due to a leak in the tank. As long as the water is not leaking into the floor it is most likely the flush valve seal, or flapper that is causing the leak. If you have the tower style flush valve that moves up, and down to flush. It is most likely due to the flush valve seal. Over time the seal will develop a curve, or bend. This will cause the flush valve seal to sit inside of the flush valve instead of sitting under the flush valve to make a proper seal. When this happens it will also cause the water to slowly leak into the bowl, which will cause the fill valve to turn on to replenish the water that was lost from the tank. This can also cause the toilet to be hard to flush. To fix the issue you will need to replace the Mansfield flush valve seal 630-0030. If you have the flapper style flush valve you will need to know if it is a 2 inch, or 3 inch flapper. If it is a 2 inch flapper the part would be the Mansfield 630-0173, and if it is the 3 inch flapper the part will be the Mansfield 630-0207.
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