The most common reason for pump failure is a power outage, not some problem with the  pump itself. Common events besides power outages can also cut off the supply of  electricity. For example, lightning can trip GFCI outlets, or someone can unplug the pump  and forget to plug it back in.

Assuming the power stays on, sometimes the pump itself fails. Many inexpensive sump  pumps are simply too small to handle the flow from a major downpour or rapid snowmelt.  And because inexpensive pumps are built with less durable materials, they lose pumping  efficiency. So the pump runs more often and burns out early. Or the motor runs but the pump  doesn’t eject water.

Float switches are also a frequent cause of pump failure. “Wide angle” tethered float  switches, the kind that free-float around the sump basket, are the biggest troublemakers.  They swirl around the sump basket, making them far more likely to get trapped against the  pump, discharge pipe or power cord. Once trapped, they can’t switch on the pump.  Inexpensive switches can also simply wear out or cause motor burnout.