Diagnosis
A clog in the drain hose or a faulty drain pump. Check the easiest point first: the air gap or sink connection.
How to Fix It
Disconnect the drain hose from the household drain under the sink. Check for a kink. Blow through it to clear any obstruction. Also check the sink-side drain connection or air gap for a clog. Reconnect securely.
If That Didn't Work
The drain pump is clogged or failed. You must access it. Unplug the unit. Remove the lower access panel. The drain pump is on the lower left side. Check the impeller for debris like glass, pit, or twist tie. If clear, the pump motor may be dead and needs replacement.
Heads Up
Never use drain cleaner chemicals in the dishwasher. They will destroy the rubber seals and the plastic tub.