Water backs up into the dishwasher from the sink when the sink is drained.

Safety first: Turn off power before working under the sink.

• Inspect drain hose routing: Ensure the hose from the dishwasher rises in a high loop under the countertop before connecting to the sink drain or disposer.
• Clear sink trap: Remove and clean the sink trap to ensure free flow.
• Install air gap (if required): In some regions, an air gap fitting is required between the dishwasher and sink drain to prevent backflow.

Correcting hose routing usually resolves backflow. No internal dishwasher parts need replacement.