A toilet may run periodically due to a problem with the flapper, the flapper seal, the flapper chain or a high float. Problems with any of these toilet parts either allows the water to continue flowing from the tank to the bowl, simulating a flush, or makes the water in the tank continuously run as it tries to fill itself.
The flapper is located in the tank of the toilet. It is the rubber stopper that is lifted by a chain attached to the flush handle. The flapper allows the water to exit the tank and rush into the bowl of the toilet. If the flapper seal is defective or worn out, the water continues to trickle into the bowl, and the flapper or seal needs to be replaced. If the flapper chain is too long, the chain may obstruct the flapper from closing all the way, which causes the same problem. If the chain is too short, however, the flapper doesn't close completely, which prevents the tank from filling completely and causes the toilet to run continuously.
If the float is positioned too high in the tank, water fills the spill tube and prevents the fill valve from shutting off. This causes the toilet to continuously run. The float is the large device within the tank that regulates the water level. It can be adjusted by means of a clip or by bending the copper rod to which it is connected.