Toilet won’t stop running: likely causes
Quick summary
If the cistern keeps refilling, first check whether water is escaping into the pan (flush seal) or out of the overflow.
Safety first
- Know where your toilet isolation valve (if fitted) or stopcock is.
- If you can’t stop the flow, isolate water supply and get help.
Full checklist: Safety guidance
What to check (in order)
- Observe: overflow vs quiet trickle into pan.
- Remove lid carefully; check water level relative to overflow.
- Lift the float (if present): does it stop the flow?
- Trickle into pan suggests flush seal/valve.
- Overfilling suggests fill valve not shutting off.
What the result means
- Float test stops flow: inlet valve behaviour.
- Overflow/high level: fill valve not closing or debris.
- Trickle into pan: flush seal issue.
What you can safely do
- Check whether water is escaping into the pan or overfilling the cistern.
- Lift the float gently, if accessible, to see whether the incoming flow stops.
- Identify the local isolation valve or stopcock before doing anything else.
- Stop and isolate the water supply if the cistern will not stop filling.
When to call a professional
- You can’t isolate supply.
- Components are concealed/unfamiliar.
- Problem persists after checks.
Engineer notes
Loss path first: overflow vs pan. For dual-flush valves, seal deformation is common. Confirm inlet valve shut-off and debris. For concealed cisterns, plan access and parts before dismantle.
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