A shutoff under a sink or beside a toilet is meant to isolate that fixture. If the valve will not turn easily or water continues at the fixture after you close it, the plumber may need to isolate the supply farther upstream.
Avoid forcing a stuck handle
Note whether the valve will move, whether water leaks around its stem, and whether the fixture still receives water. If there is an active leak and you know where the main shutoff is and can operate it safely, isolate the supply and arrange service.
Plan access before replacement
Clear space around the fixture and point out the main shutoff if you know its location. In a shared building, ask management who can authorize a supply shutdown before work is scheduled.
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