Sump Pump Diagnosis Reference

Why this matters

A failed sump pump means a flooded basement, and the customer calls during the worst rainstorm of the year. Most "sump pump won't run" diagnostic calls are actually a stuck float, a tripped GFCI, or a failed check valve - fixable in 20 minutes. Pump replacements happen, but knowing the sequence of simpler causes first saves the customer an unnecessary part swap and saves you a return trip to fix what wasn't actually wrong.

Two main pump types

Submersible (most common modern):

• Pump body sits at the bottom of the sump pit, fully submerged in water
• Sealed motor, water-cooled
• Quieter, more powerful per HP, longer-lived in continuous service
• Float, check valve, and discharge plumbing above
• Moderate residential cost

Pedestal (older / specialty):

• Motor mounted on a pedestal ABOVE the pit
• Long shaft drops down to the impeller at the bottom of the pit
• Motor air-cooled (some customers prefer for noise reduction or for service access - easier to work on without lifting)
• Less common in new installs; still found in older homes
• Lower residential cost

Discharge plumbing

• Discharge pipe size: typically 1-1/2" PVC for 1/3 to 1/2 HP pumps; 2" for larger
• Check valve: required, prevents water from falling back into the pit when the pump stops (eliminates the "back-and-forth" cycle and burnout). Spring-loaded or swing-flapper. Install vertically just above the pump.
• Discharge route: must terminate where the water won't return - yard daylight, downspout / storm sewer connection (where permitted), or in some areas a dedicated stormwater system. Never discharge to a sanitary sewer (illegal in most jurisdictions; overloads treatment plants).
• Freeze risk: outdoor terminations can freeze and back up. Install a freeze-protect discharge fitting (Sump Pump Freeze Protection, IceGuard, etc.) - has a small drain hole that lets water dribble out the side when the main line is frozen.

Float types

Tethered float (mechanical, classic):

• Ball or wedge-shaped float on a cord
• Rises with water level; at high mark, float flips and switches pump on
• Falls with water level; at low mark, switches pump off
• Reliable when working; can tangle on the pit wall, pump cord, or other plumbing
• Most common in budget pumps

Vertical float (rod-actuated):

• Float slides up and down a vertical rod
• Less likely to tangle than tethered
• Common in mid-tier pumps

Pressure switch / diaphragm:

• Senses pressure at the bottom of the pit (water depth)
• No moving float
• More reliable in narrow pits or where debris would tangle a mechanical float
• Higher-end pumps

Electronic / capacitive:

• Sensor-based, fully solid-state
• Premium / commercial typical

Common failures and quick diagnoses

Pump never runs (basin is filling):

1. Power: GFCI tripped? Look at outlet, reset. Sump pump GFCIs trip on shared circuits when the washing machine kicks on; some customers add a dedicated circuit to prevent this.
2. Float stuck: visually inspect, free if tangled
3. Burned motor: smell, hot to touch - replace
4. Switch failed: test by manually lifting the float (pump should run); if no run, switch is bad
5. Power lead disconnected from pump

Pump runs but doesn't pump (water level isn't dropping):

1. Check valve stuck closed: water can't exit; pump just churns
2. Impeller jammed: rock, toy, debris - clear
3. Discharge clogged or frozen: trace the pipe outside, find blockage
4. Pump worn: seals leaked, motor spinning but not lifting effectively - replace

Pump runs constantly:

1. Float stuck high: mechanically jammed
2. Check valve stuck open: water falls back after each cycle, pit refills, pump runs again
3. Continuous water input: sustained heavy rain OR underground spring; pump is doing its job but at peak demand
4. Switch failed in "on" position: replace

Pump cycles too frequently (short-cycle):

1. Check valve failure: water in discharge pipe falls back when pump stops; pit refills in a minute, pump starts again. Replace check valve.
2. Float positioned too close to bottom: very short travel between on and off; replace or adjust float
3. Pit too small: continuous high inflow with small holding capacity - install larger pit OR add a second pump

Pump cycles but with no water sound moving:

1. Check valve stuck closed
2. Impeller jammed
3. Pump seal failure (rotor spinning in water without lift)

Diagnostic procedure

Tools: Voltage tester, multimeter, screwdriver set, channel-lock pliers, rags, sometimes a replacement check valve.

1. Listen at the pit. Pump silent with water 6+ inches above the off-level? Power problem.
2. Verify power at outlet with a plug-in tester. If outlet dead, check breaker / GFCI.
3. Test the float manually. Reach into the pit (gloves recommended - sometimes sewage cross-contamination), lift the float. Pump should energize within 2-3 seconds. If yes, the switch is OK; the float was hanging. If no, the switch or pump is failed.
4. If pump runs but no flow: disconnect the discharge above the check valve. Pump should now push water out the open pipe. If yes, the check valve is stuck closed. If no, the pump is failed.
5. If check valve verified bad: replace (inexpensive part, 15 min labor).
6. If pump failed: plan replacement.

Replacement procedure

Time: 30-60 minutes.

Materials:

• New sump pump (size based on existing - typical 1/3 HP for typical home, 1/2 HP for high-volume basements, 3/4+ HP for finished basements with floor drainage)
• New check valve (replace at same time)
• New discharge pipe fittings if needed
• PVC primer + cement
• Sometimes new float if pump comes with a separate float

Steps:

1. Kill power at the breaker. Verify dead at the pump outlet.
2. Detach the discharge at the union just above the pump (most installs have a union).
3. Lift the old pump out of the pit. Heavy and dripping - use a tray or bucket.
4. Inspect the pit. Clean out debris that has accumulated. Make sure the new pump will sit at the correct depth.
5. Install the new pump in the pit. Connect to discharge with new fittings if old ones are corroded.
6. Install the new check valve vertically just above the pump (or wherever the original install had it).
7. Connect electrical - pump cord into the duplex outlet, or float cord on top of the pump cord (piggyback pattern for switched operation).
8. Restore power.
9. Test: fill the pit with a garden hose. Pump should start when the float trips, run smoothly, push water out the discharge, and stop when the level drops.
10. Listen for normal operation. No unusual noises, no significant vibration.

Battery backup considerations

References

• IPC 712 (sumps and ejectors)
• IRC P3303 (residential sump pumps and basins)
• Manufacturer specifications (Liberty, Zoeller, Wayne, Little Giant, Hydromatic, Basement Watchdog) - the dominant residential brands
• Local AHJ rules for discharge termination (sanitary vs storm vs daylight)