Float switches, jammed impellers, frozen discharge lines and the power cut that arrives with the storm. What to test, and how often it should really be cycling.
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Sump pumps: the part of the system that actually fails
A drainage system ends in a pump, and a pump is a motor with moving parts sitting in a wet hole. It is, by a wide margin, the most likely thing to let you down.
Everything upstream depends on one motor
A sump pump is the least glamorous and most consequential object in a wet basement. Interior drainage collects water and delivers it to a pit; from there, a single electric motor has to lift it up and out. If that motor doesn't run, the entire system is a very expensive way of bringing water to one convenient spot.
Pumps fail in ways that are boringly predictable, and most of them are visible before they matter. A float switch that hangs up against the side of the pit means a pump that never starts, or one that never stops. A discharge line that ends two feet from the foundation is pumping the same water in a circle, and one that freezes at the outlet in winter gives the pump nowhere to push. A missing or failed check valve lets the water in the vertical pipe fall back into the pit each time the pump shuts off, so it cycles again — which is the usual explanation behind "my pump keeps running". A pump that hums but doesn't move water is a different symptom entirely and usually means an impeller that is jammed or an airlock.
Then there is the failure that no maintenance prevents: the power going out during the storm that made you need the pump. These two events are correlated, which is the entire argument for a battery backup or a water-powered secondary pump. It is also the reason an alarm is worth more than most people assume — knowing the pit is filling is the difference between a nuisance and a ruined floor.
Almost everything in this section is genuinely within reach: looking at the float, checking where the discharge actually ends, testing the pump by pouring water into the pit, listening to how often it cycles. What is not in reach is rewiring a dedicated circuit or opening a sealed pump, and we say so where it comes up.
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Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.