A constantly running toilet is almost always caused by one of three things: a worn flapper, a faulty fill valve, or a float set too high. All three live inside the tank, and you can usually figure out which one is at fault in about five minutes.
The Flapper (Most Common)
The flapper is a rubber disc at the bottom of the tank. It seals the opening between the tank and the bowl. Every flush lifts it, water drains into the bowl, then the flapper drops back and the tank refills.
When the rubber ages or warps, it no longer seals fully. Water trickles into the bowl constantly, so the fill valve keeps running to compensate. You’ll hear a steady hiss or running sound even when nobody touched the toilet.
To confirm it’s the flapper: put a few drops of food coloring in the tank and wait 10 minutes without flushing. If color shows up in the bowl, the flapper is leaking.
Kohler toilets often need a specific flapper size rather than a universal one, so a plumber familiar with the model will get the right part. If a new flapper doesn’t stop the running, the flapper seat (the ring it seals against) may be pitted or corroded. That can mean replacing the flush valve assembly, which is a bigger job.
The Fill Valve
The fill valve is the tall assembly on the left side of the tank. It controls the water that refills the tank after a flush. When it wears out internally, it either doesn’t shut off completely or it cycles on and off (sometimes called “ghost flushing”).
You can often hear a fill valve problem as a higher-pitched hiss, or water that seems to run for a few seconds every 15 to 20 minutes with nobody using the toilet.
Fill valve replacement is a common plumbing call. If your toilet is a one-piece or a pressure-assisted Kohler model, the internals are different from a standard two-piece toilet and the repair requires specific handling.
The Float
The float tells the fill valve when to stop filling. On older toilets it’s a ball on an arm (ballcock style). On newer toilets it’s a cup that slides up and down on the valve body.
If the float is set too high, water fills past the overflow tube (the tall tube in the center of the tank) and drains into the bowl continuously, so the fill valve never shuts off.
You can check this yourself: lift the tank lid and look at the water level. It should sit about half an inch to an inch below the top of the overflow tube. If water is spilling into the overflow tube, the float setting is the problem. A plumber can adjust or replace the float assembly quickly, often as part of a broader tank inspection.
How the Diagnosis Works
A plumber checks the same things you can see: the water level relative to the overflow tube, the dye test for the flapper, and whether the fill valve is cycling. From those observations they know what to replace. On a toilet that’s been neglected for a while, many plumbers replace the whole internal kit (flapper, fill valve, float assembly) at once, since the rubber ages together and a second trip for a different part costs more than doing it right the first time.
Call a Licensed Plumber
The repairs described here all involve shutting off the supply valve, draining the tank, and working inside it. Some shutoff valves on older toilets are stuck and will start leaking if forced. The flush valve seat can be corroded in ways that aren’t visible until you’re already in the middle of the job. Pressure-assisted toilets use a pressurized vessel inside the tank and need specific parts and handling, not a standard fill valve swap.
If there’s water on the floor around the base, that’s a separate problem (wax ring or supply connection) and it needs prompt attention regardless.
A licensed C-36 plumber will have the right parts, handle surprises without creating a bigger problem, and stand behind the work. Verify any plumber’s license before hiring at cslb.ca.gov.
This site is an informational guide only. We do not offer plumbing services.
The Short Version
The dye test tells you if the flapper is leaking. Water spilling into the overflow tube means the float is off. Cycling or hissing with nobody using the toilet points to the fill valve. Once you’ve identified the cause, the right move is a licensed plumber, not a hardware store run.