Symptoms
- Toilet runs for a few seconds every 20–60 minutes
- No one has touched it
- Happens more at night when it's quiet
- Water level in tank is slightly low before it refills
Likely causes
- CommonFlapper seeping slowlyA worn or warped flapper lets a slow trickle into the bowl until the tank senses low water and tops itself off.
- CommonMineral buildup on the flush valve seatGrit under the flapper prevents a full seal even if the flapper itself looks fine.
- PossibleFill valve slowly leaking past its sealA worn fill valve can weep water into the tank, changing the level enough to trigger a top-off.
What to do
- Remove the tank lid and mark the water level with a pencil, then check it again in 20–30 minutes without flushing.
- If the level dropped, do a food coloring dye test in the tank to confirm color appears in the bowl.
- Feel the flush valve seat (where the flapper sits) for grit, mineral buildup, or a rough edge.
- Clean the seat gently with a soft cloth and vinegar if buildup is present.
- Replace the flapper with a new universal one if cleaning doesn't fix it.
- Recheck water level after a few hours to confirm the phantom flushing has stopped.
Tools and parts
- Gloves
- Vinegar
- Soft cloth
Universal toilet flapperSearch: “universal toilet tank flapper replacement”
When to call a pro
If a new flapper doesn't stop it, the flush valve seat itself may be damaged and a plumber can assess whether it needs replacing.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.