Sink leaking underneath, (leak changes when faucet runs)

Use only checks that match the exact appliance or system and its official guide.

1. Check 1

   Dry the cabinet and observe from a safe position with the faucet off, then open it briefly and close it; record whether the first moisture appears while filling, after shutoff, during draining, or with no use.

   Expected result: Record a leak only during faucet use, only while the basin drains, a continuous leak with the faucet off, or no new moisture during one controlled test.

   Next step: Repeat only once with a dry towel at likely joints, then inspect the first wet point. Stop using the sink if water reaches the cabinet floor or electrical equipment.

   Stop if: Stop for a fast stream, a split hose, a valve that will not close, water near electrical equipment, cabinet swelling, or a ceiling or wall stain below the sink.

   Why this matters

   Why: Use-related timing favors a faucet hose or drain path; drain-only timing favors the drain assembly; a continuous leak favors a valve or supply line. No new moisture means the source may be intermittent or elsewhere.

2. Check 2

   Place dry paper towels separately beneath the faucet supply connections, shut-off valves, drain flange, trap joints, and any dishwasher or disposal connection, then run a small amount of water while watching the towels.

   Expected result: Record the first towel to dampen, water tracking from above, water appearing only after the drain flows, or water arriving from a hose or valve that stays wet with the faucet off.

   Next step: Photograph or mark the first wet point and stop the test. Arrange plumbing service if the point is not a simple, hand-accessible user connection identified by the manual.

   Stop if: Do not put hands into a disposal, disconnect a hose, reach behind electrical equipment, or loosen a fitting to see whether it leaks.

   Why this matters

   Why: The first wet point is more useful than the puddle location. A supply or faucet connection is pressurized, while a drain or trap normally shows water when the basin empties.

3. Check 3

   With towels in place, fill the basin slightly without using the faucet again, wait briefly, and then release the water while observing the drain flange and trap from a safe position.

   Expected result: Record moisture while the basin is full, moisture only as water drains, no moisture at the drain but a wet faucet hose, or a leak that continues after the basin is empty.

   Next step: Leave the drain area dry and the sink unused if the leak returns. Request plumbing service for a flange, trap, concealed drain, or persistent supply leak rather than applying sealant over the wet area.

   Stop if: Do not remove the drain flange, disassemble a trap containing unknown water, or use the sink if the cabinet is flooding.

   Why this matters

   Why: A flange or basin seal can leak while water is held or drained; a trap joint often shows during discharge. A leak unrelated to basin water points back toward a supply or valve.

4. Check 4

   From a dry position, look at the shut-off valve body and handle, the supply line, and the faucet connection for a split, corrosion, bulge, or moisture that remains when the faucet is untouched; operate a valve only if it turns normally and does not require force.

   Expected result: Record a dry valve and line, a steady drip at the valve or supply line, a damaged or bulging hose, a valve that will not close, or a connection that leaks only when the faucet is used.

   Next step: If the valve operates normally, close it to stop a controllable leak and leave the faucet off. If it will not close or the line is damaged, stop using the sink and call a plumber; do not force the handle.

   Stop if: Do not disconnect a pressurized hose, overtighten a nut, use pliers on a stuck valve, or continue if water sprays or the valve body moves.

   Why this matters

   Why: Moen distinguishes steady valve or supply leaks from intermittent faucet-connection leaks. A damaged hose or valve can worsen suddenly, and a valve that will not isolate the fixture needs prompt professional help.

5. Check 5

   Look for water entering from the sink rim, overflow path, disposal, dishwasher hose, water filter, or cabinet wall while the faucet runs; keep hands and tools away from disposal blades and electrical connections.

   Expected result: Record a leak at an accessory connection, water at the overflow or rim, a disposal or dishwasher hose leak, or moisture that appears to come from the wall or ceiling.

   Next step: Stop using the affected fixture or appliance and arrange plumbing service. If water is near electrical equipment, keep clear and use the appropriate electrical or emergency response.

   Stop if: Do not operate a disposal with water present near its electrical parts, pull appliances out, open a wall, or enter a flooded cabinet.

   Why this matters

   Why: A connected appliance or overflow can mimic a faucet or drain leak. Water from a wall, ceiling, or energized appliance area exceeds a simple faucet adjustment and may damage the building.

6. Check 6

   Record the first wet point, whether the faucet was on or the basin was draining, the leak rate, valve response, visible hose or seal condition, sink and faucet model if safely available, and any water damage.

   Expected result: Confirm that the record contains external observations only and that no supply line, drain, trap, valve, disposal, or concealed connection was disturbed.

   Next step: Request plumbing service and provide the timing, photos, model information, and whether closing a normal shut-off valve stopped the leak.

   Stop if: Stop all user troubleshooting when the leak requires disconnection, force, concealed access, electrical isolation, or work inside a wall or ceiling.

   Why this matters

   Why: Timing and location let a plumber distinguish a faucet connection from a shut-off valve, supply line, drain flange, trap, overflow, or concealed leak without unnecessary disassembly.