What Usually Causes It
Most running toilets are caused by one of a few common issues inside the tank. The first thing to check is the flapper. This is the rubber piece at the bottom of the tank that lifts when you flush and then drops back down to seal the water inside.
If the flapper is worn, bent, dirty, or not sitting flat, water can keep leaking into the bowl even after the flush is finished. That small leak is often what causes the tank to refill again and again.
Another common issue is the chain connected to the flapper. If the chain is too short, twisted, or caught on something, it may stop the flapper from closing all the way. Even a small gap can keep the toilet running.
The float can also be the problem. The float controls how high the water rises in the tank. If the water level is too high, it may spill into the overflow tube and cause the toilet to keep running.
The good thing is that all of these problems are easy to check by simply lifting the tank lid and watching what happens after a flush.
