A washing machine appliance includes a sump for collecting wash fluid, a water supply valve for supplying wash fluid during a fill cycle, and a drain pump assembly for draining that wash fluid during a drain cycle. A controller is configured for monitoring the sump pressure using a water level detection system, detecting a fill sensing failure such as a slow pressure rise after the fill cycle, detecting a drain sensing failure such as a slow pressure drop during the drain cycle, and determining that the water level detection system is malfunctioning if the fill sensing failure and the drain sensing failure are detected.
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