Quick Answer:
If the shutoff under the sink won’t stop water, first isolate the supply to limit damage, then inspect the valve and try to remove any debris. If the valve is corroded or the stem is broken, replace the angle stop. If you can’t safely stop the flow, shut off your home’s main water and call a plumber.
Why This Happens
Angle stops are small, inexpensive valves and fail for a few common reasons:
- Internal parts wear out — washers, seats, or the valve stem can degrade and stop sealing.
- Mineral deposits or grit get stuck in the valve, preventing it from seating properly.
- Corrosion or seized stems make the handle spin without closing the passage.
- Loose packing nuts or damaged compression fittings let water bypass the valve body.
When a valve won’t close you may also notice continuing flow or a persistent drip once it’s reopened — a symptom similar to a **shutoff valve dripping** problem caused by worn sealing surfaces.
Step-by-Step What to Do
Step 1 — Stop water flowing (isolate the supply)
Act fast to limit water damage.
- Try the angle stop handle gently to confirm it won’t close. Don’t force it.
- If it still won’t close, shut off your home’s main water valve or the building’s service valve. If you live in a multiunit building, contact maintenance if you can’t locate a main shutoff.
- Place towels and a bucket under the sink to catch any drips while you work.
Step 2 — Inspect the valve
With the supply isolated:
- Look for obvious damage: a broken handle, green or white corrosion, or a loose packing nut.
- Try turning the valve handle gently back and forth a short distance to see if the stem moves at all — this tells you if the stem is seized or stripped.
- Check the connection to the flexible supply line for leaks where it joins the valve.
Step 3 — Flush debris (safe cleaning)
If the valve looks intact but won’t seal, trapped debris is a common cause.
- Turn the house main back on briefly only if the supply line is disconnected from the valve and you have a bucket to catch the water — this will flush grit out of the line. Otherwise keep the main off.
- Alternatively, remove the supply line and inspect it for sediment. Clean or replace the line if clogged.
- After flushing, retest operation with the main on and the supply line reattached (watch for leaks).
Step 4 — Replace the angle stop if needed
If the valve is corroded, the stem is stripped, or it still won’t seal after flushing, replacement is the safest fix.
- Turn off the main and drain the lines by opening a faucet.
- Remove the supply line nut and the valve (compression, sweat, or threaded connection). Note how it’s connected — you’ll need the same type of new valve.
- Install a new angle stop following the fitting type. If you’re unsure about soldering or pipe threads, hire a pro.
- Turn water back on slowly and check for leaks.
What Not to Do
- Do not force the valve handle with pliers or hammer it — forcing can break the stem and make the problem worse.
- Do not ignore a leaking or non-closing valve — small leaks become big water damage quickly.
- Do not attempt major pipe soldering or cutting without the right tools and experience; shut off water and call a plumber if you’re unsure.
When to Call a Professional
- If you cannot find or close the main shutoff quickly.
- If the valve breaks when you try to operate it (broken stem) or a fitting starts spraying water.
- If the connection type requires soldering or the valve is on hard-to-reach copper or iron supply lines.
- If you notice similar issues on other fixtures — for example, recurring problems like **Toilet shutoff leaks** after you work on one valve may indicate systemic water-quality issues or old plumbing that needs professional attention.
Safety Notes
- Always shut off water before removing fittings. Have a bucket and towels ready.
- Turn off nearby electrical devices or the circuit breaker for outlets under the sink if water is present.
- Wear gloves and eye protection when working under the sink; corrosion and mineral deposits can be sharp or irritating.
- If you smell gas or see a major burst, evacuate and call emergency services or your utility provider.
Common Homeowner Questions
- Can I use a pipe wrench to force the valve closed? — No. Forcing often breaks the valve stem and makes repairs harder.
- Will a new angle stop fix a slow leak under the sink? — Often yes, if the leak comes from the valve itself; confirm the source before replacing.
- How long does replacing an angle stop take? — A straightforward replacement can be 20–60 minutes for a DIYer; allow more time if fittings require sweating or if corrosion is heavy.