Water backs up into the dishwasher from the sink or garbage disposal, leaving dirty water in the tub.

Safety first: Do not run the dishwasher while the sink is backing up; this can contaminate dishes and cause overflow.

Step-by-step checks:

• Sink drain: Clear any clogs in the sink or disposal using mechanical means (plunger, snake) rather than chemicals.
• High loop: Ensure the dishwasher drain hose is routed up under the countertop before dropping to the sink drain or disposal.
• Air gap: If installed, remove the cap and clean the air gap of debris.
• Disposal connection: Confirm the hose is attached to the correct port and that the disposal knockout was removed at installation.

Warning: Chemical drain cleaners can damage dishwasher components if they backflow into the unit.