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    plumbing troubleshooting

    Fix running toilet

    Common symptoms: running toilet; phantom flush; toilet keeps refilling; flapper leak; fill valve runs

    Stop and call a pro if:

    • water on floor near electrical devices

    Step-by-step diagnostic flow

    1. Step 1

      Is there water on the floor near electrical outlets or devices?

    2. Step 2

      What pattern best describes the running?

    3. Step 3

      Add a few drops of food coloring to the tank (not the bowl). Wait 10 minutes without flushing. Does color appear in the bowl?

    4. Step 4

      Remove the tank lid. Where is the water level relative to the top of the overflow tube?

      Do not adjust anything yet — just look.

    5. Step 5

      How old is the fill valve, and is it the original brand still made (e.g., Fluidmaster 400A/PerforMAX)?

    Possible outcomes

    Stop and seek professional help immediately

    high confidence

    Potential electrical hazard with water present.

    Safe next steps
    • Kill power to affected circuits at the breaker
    • Shut off water at the toilet angle stop
    • Call a plumber
    Electrical or safety hazard present

    Flapper or flush-valve seat is leaking — replace the flapper

    high confidence

    Dye reached the bowl without flushing, confirming tank water is bypassing the flapper.

    Safe next steps
    • Shut the angle stop and flush to drain the tank
    • Take the old flapper to the store to match brand/model (or photograph the flush valve)
    • Install a matching flapper, set chain slack to 1/2 inch
    • Turn water back on and re-run the dye test to confirm
    What to document for a pro
    • Toilet brand/model (inside tank stamp)
    • Photo of the flush valve

    Fill valve set too high — water is overflowing into the tube

    high confidence

    Water at or above the overflow tube means the fill valve never gets a shutoff signal because water keeps draining through the overflow.

    Safe next steps
    • Adjust the float down (clip or screw on the float stem) so the level sits about 1 inch below the top of the overflow
    • Flush and confirm the valve shuts off cleanly
    • If the float will not hold a lower setting, replace the fill valve

    Fill valve set too low — weak flush, may be unrelated to running

    medium confidence

    A very low tank level reduces flush volume and may explain bowl issues but does not by itself cause running.

    Safe next steps
    • Adjust the float up to set water 1 inch below the top of the overflow
    • Re-evaluate whether the toilet still runs after the level is corrected

    Likely a worn fill valve diaphragm — replace the fill valve

    medium confidence

    Older fill valves develop slow internal leaks that cause hissing or short refill cycles even at a correct water level.

    Safe next steps
    • Shut the angle stop, flush to drain, sponge out residual water
    • Disconnect supply line and remove the old fill valve
    • Install a universal fill valve (e.g., Fluidmaster 400A) per the instructions, set water level 1 inch below overflow
    What to document for a pro
    • Toilet brand/model
    • Photo of existing fill valve

    Further diagnosis recommended

    low confidence

    Recent replacement or proprietary parts mean common DIY swaps may not apply.

    Safe next steps
    • Photograph the tank interior, fill valve, and flush valve
    • Note when the issue started and any recent work on the toilet
    What to document for a pro
    • Toilet brand/model
    • Fill valve brand/model
    • Photos of tank components
    Diagnostic guidance only. If unsure, stop and call a licensed professional — gas, electrical, and refrigerant work is hazardous to untrained users.

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