Sump pumps run continuously for many reasons; however, if the pump was operating normally and the problem is recent, the most likely cause is a stuck float switch. This switch moves up with rising water levels to complete the circuit and start the pump. It then moves back down with the water level to break the circuit and shut off the pump, but obstructions prevent normal operation.
A missing or inoperative check valve also causes a pump to continue to run. The check valve prevents water from draining back into the sump from the discharge line. If it is not operating correctly, the cycling of water never allows the sump to empty, even though the pump runs without stopping.
Sump pumps that are too small to handle the amount of water collecting in the sump do not stop running. Installing an undersized pump causes excess heat and wear, reducing the life of the device. The undersized pump does not provide adequate protection for the basement, and flooding continues to be a problem.
In rare cases, there is a continual flow of water into the sump. Sumps that extend into the water table cause the pump to run and not stop. Lifting the pump a couple of inches allows it to cycle normally while protecting the basement.