Fix a Running Toilet
A practical homeowner guide to diagnosing a toilet that runs or refills by itself. Follow the safety checks, confirm the exact part or model, and test your work before considering the repair complete.
Before you start
Look for corrosion, moisture staining, and the condition of nearby shutoff valves before taking anything apart. Photograph the original connection so you can restore the arrangement correctly.
Tools and supplies
- Food coloring
- replacement flapper
- towel
Tools & Materials for This Project
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Also prepare eye protection, a clean workspace, and any product-specific tools or fittings listed in the installation manual.
Step-by-step: Diagnosing a toilet that runs or refills by itself
Read the entire procedure before starting. Use the product manual and local code requirements where they are more specific.
- Remove the tank lid and observe whether water is flowing into the overflow tube or leaking from tank to bowl.
- Mark the tank water level, shut off the supply, and wait 15–20 minutes; a falling tank level points toward a leaking flapper or flush-valve seal.
- If water is above the overflow opening, adjust the float to the fill valve’s marked water line; clean or replace the fill valve if it does not shut off.
- If water falls below the marked line with the supply off, inspect the flapper/flush-valve seal for mineral deposits, warping and poor fit.
- Check chain slack: it should lift the flapper freely without holding it open. Use the correct replacement for the flush-valve model.
- Turn water on, flush several times, and confirm the valve shuts off and the tank level stays stable.
Common mistakes to avoid
- Do not replace the fill valve when the actual problem is a leaking flapper.
- Avoid chlorine drop-in tablets in the tank; they can damage seals and rubber components.
- Do not set the fill level above the overflow tube.
Helpful video tutorials
Watch a relevant demonstration before starting. Check the model, safety steps and local code requirements for your project.
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DIY or hire a professional?
A plumber should handle inaccessible piping, failed shutoff valves, recurring leaks, or any damaged subfloor. A professional quote should explain labor, materials, disposal, warranty, site conditions and any permits. Compare identical scopes, not just total prices.
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