Sump Pump Hums but Does Not Pump

A humming pump that is not moving water can have several possible causes, including an obstructed intake or impeller, a restricted discharge line, a stuck mechanical component, or a motor/start-component problem. The sound alone does not identify the exact failure.

Jammed Impeller

Disconnect power before any hands-on inspection. Check only accessible areas permitted by the manufacturer’s instructions for visible debris or obstruction. Do not reach into an energized pit or disassemble the pump unless the manufacturer provides a safe procedure and you are qualified to perform it. Persistent humming without pumping warrants service evaluation.

Seized Motor

Disconnect power before any hands-on inspection. Check only accessible areas permitted by the manufacturer’s instructions for visible debris or obstruction. Do not reach into an energized pit or disassemble the pump unless the manufacturer provides a safe procedure and you are qualified to perform it. Persistent humming without pumping warrants service evaluation.

Failed Start Capacitor

Some sump pump motors use a capacitor to provide the extra power needed to start spinning. If the capacitor has failed, the motor may hum because it’s receiving power but lacks the initial torque to start turning. This is more of a component-level electrical issue and is generally something a professional should diagnose and replace rather than a DIY fix, since it involves the pump’s internal wiring.

Airlock in the Pump Housing

Air trapped inside the pump casing can prevent water from being drawn in properly, causing the motor to run without effectively pumping. This is more common after a pump has sat unused for a while or after reinstalling one. Removing the pump, tilting it to release trapped air, and reinstalling it can sometimes resolve an airlock without further repair.

Diagnosing the Cause

1. Unplug the pump and remove it from the pit.
2. Try turning the impeller by hand—if it won’t move, look for debris; if it spins freely, the issue is likely the motor or capacitor.
3. Check for water actually entering the pump housing; if it seems dry or resistant, air may be trapped inside.
4. If the impeller moves freely and no air or debris issue is found, the motor or capacitor is the likely cause.

Repair or Replace

A jammed impeller or airlock is typically a quick fix. A seized motor or failed capacitor is harder to repair in the field and often makes replacement more practical, especially on an older pump. See Sump Pump Repair for general troubleshooting steps, or Sump Pump Replacement if the pump is older or has failed before.

If the pump makes no sound at all rather than humming, see Sump Pump Not Working instead. For the full range of sump pump issues, see the Sump Pump Problems Guide.