The short answer: identify where the water is going
To fix a running toilet, first distinguish a leaking flush seal from a fill valve that will not stop. In a conventional gravity toilet, the fill valve brings water into the tank; the flapper or flush-valve seal holds it there until you flush. A stuck handle, tight chain, worn seal or high water level can keep that cycle going. Start with observation before buying replacement parts. Lift the porcelain lid with both hands and place it on a towel where it cannot fall. Watch a flush, then note whether water continues entering the overflow tube or whether the tank becomes quiet and refills later. Have towels, a sponge and a bucket ready; Kohler’s preparation guide also lists an adjustable wrench when a repair requires one. This guide concerns ordinary gravity tanks. A sealed pressure vessel, concealed cistern or electronic toilet needs its own service instructions. If water is escaping onto the floor, close the supply valve if it operates normally and arrange repair; an internal running-water diagnosis does not explain an external leak.
Use a dye test to check the flapper or flush seal
A toilet that briefly refills when nobody has flushed may be replacing water lost from the tank. With the tank full, add a few drops of food coloring or a toilet leak-detection tablet, then leave the toilet unflushed for 10 minutes. If color appears in the bowl, water is passing from the tank into it; the EPA’s toilet-maintenance guidance identifies a worn flapper as a common cause. Flush afterward to prevent staining. Also check whether the handle returns freely and the chain allows the flapper to settle completely. A chain trapped underneath the seal can defeat an otherwise good part. Shut off the supply and flush before handling or replacing a seal. Photograph the tank’s model number and existing mechanism, then match the replacement to that model: similar-looking seals are not necessarily interchangeable. A canister-style flush valve may use a replaceable ring rather than a hinged flapper. Gently remove loose debris from the sealing surfaces, fit the correct part according to its instructions, restore water and repeat the test. Record the result so you can distinguish a successful seal repair from a separate filling problem.
Check the float, water level and refill tube
If the tank keeps filling into the overflow, concentrate on the fill valve and float rather than assuming the flapper is faulty. Check that the float moves freely and is not catching on the tank wall or another component. Adjust the water level using the mechanism specified for your valve and the toilet manufacturer’s marked fill level. For one specific Fluidmaster repair kit, its installation and troubleshooting instructions specify a level ½ inch below the overflow pipe; that example is not a universal setting for every toilet. The small refill tube matters too: Fluidmaster’s refill-tube instructions warn against pushing it down inside the overflow pipe, which can waste water. Use the intended clip and arrangement for your model. If the valve keeps admitting water after its float reaches the proper shutoff position, consult its service procedure or replace it with a compatible assembly. Close the supply and drain the tank first. Do not open a valve cap while it is pressurized, overtighten plastic fittings or disturb the valve’s backflow-protection setup.
Verify the repair and know when to call a plumber
After reassembly, reopen the supply slowly and watch the entire refill. Flush again, confirm that the seal closes and the water stops at the intended level, then inspect the supply connection, tank underside and surrounding floor for moisture. Wipe connections dry before checking them so old drips do not confuse the result. Finally, repeat the dye test and listen for refilling after the bathroom has been unused. Success means a complete normal cycle and no continuing leak, rather than silence achieved by leaving the supply shut off. If the shutoff valve is seized, a tank is cracked, a connection leaks or a correctly matched new seal still fails, stop and get a plumber’s diagnosis. Replacing several unrelated parts at once can make the original fault harder to identify. Keep a simple maintenance note with the toilet model, replacement part and repair date; the EPA recommends checking for silent leaks at least annually. If you are reviewing water-use figures, remember that a U.S. gallon is about 3.785 liters; a unit conversion helps compare records, but it cannot establish how much this particular toilet was leaking without a measurement.
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