When a sump pump emits a loud humming sound but fails to pump water or spin, it indicates that electrical power is reaching the motor, but the impeller is jammed by debris, the motor bearings have seized due to wear, or the unit is air-locked. An air lock occurs when an air pocket becomes trapped in the pump housing, preventing the impeller from priming. Callout type="warning" title="Safety Warning": Unplug the pump before servicing. Verification: Check the impeller for debris and tilt the pump slightly or drill a tiny anti-airlock hole in the discharge pipe above the check valve.
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