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Yes, a dishwasher needs an air gap to prevent backflow. Additionally, in many locations, air gaps or another measure to prevent backflow are required for building code compliance. There are alternatives to an air gap, like a Studor vent or a loop and trap design, but an air gap is the simplest and most popular solution.  

If the dishwasher is in working condition, start and run a complete cycle with nothing inside. Let it drain and sit overnight before removing the hoses. In theory, it should effectively drain. You may find residual water in the hoses, but you can plug them. If the dishwasher breaks down midcycle it can sometimes be full of water. You’ll need to manually remove the water with a plastic cup before removing the dishwasher.

To create a siphon break and remove any chance of dirty sink water or water that backs up into the sink from getting into your dishwasher, your dishwasher drain hose needs to have a high loop or an air gap device as part of its installation. All building codes require at least a high loop. In many places, it’s necessary to install an air gap device to meet the code.

It might be time to buy a new dishwasher for your kitchen if you consistently notice a significant amount of water left over after a cycle, indicating potential drainage issues that are becoming persistent or costly to fix. Visible rust, especially on the interior tub or racks, suggests internal corrosion that will likely worsen over time and can compromise cleaning performance. Additionally, if your dishes are coming out excessively hot after the drying cycle, it could point to a malfunctioning heating element or thermostat, which can be an energy inefficiency and a potential safety concern, making replacement a feasible choice.