Leak appears only when the fixture runs: supply line or drain?

Could you add one isolation step before testing the fill cycle: shut off the fixture’s supply valve, then drain only whatever water is already in the bowl, basin, or appliance? If it still leaks while that water is draining, that points more toward the drain seal, trap, or downstream connection than a pressurized supply or inlet hose.

If it stays dry with the supply shut off but leaks when the valve is reopened and the fixture fills, inspect the supply-side joints and inlet connection first. Keep the test brief, and don’t loosen any pressurized fitting unless the water is shut off upstream.

Test the overflow separately by drying the area, then adding water slowly until it reaches the overflow level without running the fixture or opening the drain. If it leaks only during that test, the likely area is the overflow gasket, cover, or the fixture around that opening rather than a pressurized supply connection or drain trap. Stop as soon as the first paper towel shows moisture, and keep water from reaching flooring or cabinetry.

If the first moisture shows up only while draining, don’t limit the check to the trap. Inspect the drain flange and gasket above it too. A leak at that seal can run along the underside of the fixture or cabinet before dripping from the lowest trap joint, making the trap look guilty.

After drying everything, place separate towels under the flange, each trap connection, and the downstream joint. Watch for the first towel to dampen while draining a small amount of water. A slip-joint nut may only need careful hand-tightening, but avoid tightening a corroded or cracked fitting aggressively. If the flange or gasket is leaking, or water is reaching cabinetry or flooring, stop using the fixture until it’s repaired.

A cracked fixture body or inlet/drain hose can leak only while water is moving, especially if vibration or the weight of water opens the crack. For a similar leak, run a small amount of water and inspect the fixture underside and hose ends with a flashlight, supporting the hose without sharply moving it. Separate towels under the fixture body and hose can help distinguish those leaks from the flange or trap during a brief fill-and-drain test.

When checking under a fixture, remove a toe-kick or access panel only if it comes off safely and won’t expose wiring or moving parts. A concealed splash, or an inlet hose rubbing against the cabinet, can make a drip appear to come from the trap or the fixture’s lowest edge. Keep the test brief, and don’t pull or sharply bend a hose just to improve the view.

For the appliance-hose possibility, does the leak begin during filling, or only when the pump is running? That can help distinguish an inlet-side connection from a pump-side hose or seal. Which appliance is it? The layout varies, so the likely leak point depends on the model.