How can I tell if a running toilet is caused by the fill valve or the flush valve?

First, check where the water is leaking. There are two common situations: water leaking onto the bathroom floor, or water continuously flowing into the toilet bowl.

If water is leaking onto the floor, bend down and touch around the tank and connections to locate the leaking point. For a one-piece toilet, where the tank and bowl are integrated, floor leakage is usually caused by the fill valve inlet connection. For a two-piece toilet, check whether the leakage comes from the fill valve inlet, the flush valve outlet, or the tank-to-bowl connection.

If water keeps flowing into the toilet bowl, open the tank lid and check the water level. If the water level reaches the top of the overflow pipe, the problem is usually related to the fill valve. The fill valve may fail to shut off, or the float may be adjusted too high. In this case, adjust the fill valve height or float height first. If it still cannot stop water, the fill valve may need to be replaced.

If the water level is below the top of the overflow pipe but water still leaks into the bowl, the problem is usually with the flush valve seal. The seal may be aged, deformed, or not sitting properly. You can replace the correct sealing gasket, or replace the complete flush valve if necessary.

For two-piece toilets, there is also one hidden leakage point: the bolts between the tank and the bowl. If the fill valve, flush valve, and tank-to-bowl gasket all seem fine, remember to check the tank bolts as well.