Toilet Keeps Running: Diagnose and Fix It Step by Step

How Your Toilet Tank Works (the 60-Second Version)

You need to understand the three key components before you can diagnose anything.

• The flapper: A rubber seal at the bottom of the tank that holds the water in until you flush. When you push the handle, it lifts, lets water rush into the bowl, then drops back and seals. When the flapper does not seal tightly, water continuously trickles from the tank into the bowl.
• The fill valve: Attached to the left side of the tank. When the tank drains after a flush, the fill valve opens to refill it. When the water reaches the correct level, it shuts off. A worn fill valve may not shut off cleanly.
• The float: Connected to the fill valve, the float (either a ball on an arm or a cup float on the fill valve shaft) rises with the water level and signals the fill valve to stop. If the float is set too high, water rises above the overflow tube and drains continuously.

The Dye Test: Confirm a Flapper Leak in 15 Minutes

Before disassembling anything, do this test:

1. Remove the tank lid and set it safely aside.
2. Add a few drops of food coloring (any color) or a dye tablet into the tank water.
3. Do not flush. Wait 15 minutes.
4. Look into the bowl without flushing. If color has appeared, the flapper is leaking.

If no color appears in the bowl, the flapper is sealing correctly and your issue is with the fill valve or float level.

Fix 1: Replace the Flapper

This is the most common running toilet repair. Replacement flappers cost $5 to $15 and take about 10 minutes to install.

1. Turn off the water supply at the shut-off valve behind the toilet (clockwise to close).
2. Flush the toilet to empty the tank.
3. Unhook the flapper. Most flappers hook onto pegs on the sides of the overflow tube and have a chain connected to the flush handle arm. Unhook both.
4. Take the old flapper to a hardware store or note the toilet manufacturer to find the correct replacement. Flappers are not universal — using the wrong size causes the same problem.
5. Install the new flapper, hook the chain to the flush arm (leave a small amount of slack — about an inch), turn the water back on, and test. The toilet should flush cleanly and stop running within 60 seconds.

Fix 2: Adjust or Replace the Fill Valve

If the dye test showed no leak and the toilet is still running, watch the water level in the tank while it fills. If water is rising to the top of the overflow tube (the tall vertical tube in the center of the tank) and running down into it, the float is set too high or the fill valve is not shutting off.

Adjust the Float First

• Ball float (ball on a long arm): Bend the arm slightly downward so the ball sits lower in the water and triggers shutoff sooner.
• Cup float (slides along fill valve shaft): Pinch the clip and slide the float down the shaft, then test. Most cup floats also have a screw adjustment at the top of the fill valve.

The water level should sit about 1 inch below the top of the overflow tube.

Replace the Fill Valve If Adjustment Does Not Help

A fill valve that has worn out internally may not shut off regardless of float position. Replacement fill valve kits (Fluidmaster 400A is a common reliable choice) cost $10 to $20 and include instructions. The replacement takes about 20 minutes with pliers.

What to Do If the Toilet Still Runs After Both Fixes

If you have replaced the flapper and adjusted or replaced the fill valve and the toilet is still running, the issue may be:

• A crack in the overflow tube causing improper water levels.
• The flapper seat (the plastic ring the flapper seals against) is warped or damaged — the seat cannot be resurfaced and requires the flush valve assembly to be replaced, which involves draining and partially dismounting the tank.
• A corroded or damaged internal component specific to your toilet model.

At this point, calling a plumber is the most efficient path. If your toilet is old and has needed multiple repairs, a full replacement may be more cost-effective than further repairs.

When to Call a Licensed Plumber for Toilet Repair

• You replaced the flapper and fill valve but the toilet still runs.
• The toilet is rocking or the base is wet (possible wax ring failure).
• The toilet is cracked or the tank has cracks.
• Water is backing up into the bowl from elsewhere in the house (main line blockage).
• You are not comfortable working on the internal components.

Alberto Plumbing handles all toilet repairs and replacements throughout Pflugerville, Round Rock, Austin, and Hutto. Call (512) 429-6933 for same-day service — Jose A. Vital (TX Master Plumber M-39647) provides upfront pricing and carries common toilet repair parts on the truck.