• First things to know are if your toilet is constantly running the problem resides within the tank and to shut down your water line before beginning any plumbing work.
• Check the fill tube that runs from the fill valve on the tank’s left side to the overflow tube’s right. Make sure the fill tube is tightly attached to the fill valve and that water is flowing from the valve to the tube without any interruption. The fill tube also shouldn’t extend into the pipe if it does trim it accordingly.
• Check the float if it’s too high or low. The float is a cup shaped plastic piece attached to the valve that notifies the valve about the correct water level of a tank. Adjust the float using the attached clip, rod, or screw until the correct water level is restored, which should be marked on the inside of the overflow tube or the tank.
• Check the handle. The handle of a toilet is connected to the flush rod , a plastic piece attached to a chain leading to a flapper. Make sure the chain has enough slack to let the flapper open, and that the flush rod does not strike the inside of the tank lid when flushed.
• Check the flapper and replace if required. The flapper is a seal that moves when you operate the flush handle, it lets water in and out. You must drain the water from the tank to inspect the flapper Use the shut off valve to stop the water flow. Then dry up the remaining water in the tank. If there’s a crack in the rubber seal or it doesn’t fit properly it needs to be replaced.Unclip the old flapper from the overflow tube, flush the rod chain and replace it with a new one.

Replace everything but the toilet. If you have followed all the steps above and it’s still running there is something significantly. You should remove and replace the flapper, fill valve, and other accompanying parts.