Water backs up into the dishwasher from the sink or garbage disposal.

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

• Clear sink drain: Ensure the sink and garbage disposal drain freely. Clear any clogs.
• Verify high loop: Make sure the dishwasher drain hose is routed up under the countertop (high loop) before connecting to the drain or disposal.
• Install air gap (if required): In some areas, an air gap is required. Install one if missing and local codes demand it.
• Check valve: Inspect the dishwasher's internal check valve (if accessible) for debris and ensure it closes properly.