There are a number of different things that can result in a dishwasher not draining, which can be broken down into general categories. Many dishwasher drainage issues can be traced back to the inlet or outlet hoses. It is also possible for a dishwasher to not drain properly due to a garbage disposal, particularly if a new unit was just installed. Other issues that can result in a dishwasher not draining are related in some way to the pump, solenoid, or timer. Some of these issues are easy for anyone to inspect and repair, though in some cases a professional will need to be contacted.
A dishwasher that is permanently installed underneath a counter or sink is typically connected to a water inlet source and a drain. Problems with either of these hoses can result in the dishwasher not draining. If the drain hose is kinked, it can cause water to back up into the dishwasher during the drain cycle. It is also sometimes possible for water to leak into the dishwasher through the inlet hose when the unit is not in use. A kink in the drain can be suspected if the dishwasher is full of water immediately after use, while a leaking hose may be the culprit if there is water present before the wash cycle begins.
Another related cause for a dishwasher not draining can be traced back to a garbage disposal. Most dishwashers connect to the sink drain using a special connector, but a garbage disposal can make this configuration difficult. Instead, many garbage disposals include a dishwasher drain connector. From the factory, this tube is blocked with a knockout plug that has to be removed prior to connecting the dishwasher. If the knockout plug is not removed, the dishwasher will not be able to drain and dirty water will back up into it.
Many other dishwasher drainage issues are related in some way to the pump. This component is meant to pump the used water out of the dishwasher and into the sink drain or disposal. When a pump fails to reverse when it is time to begin removing the water, it can result in the dishwasher not draining properly. If the dishwasher fails to pause in between each washing and draining cycle, the pump can be suspected. There may be a solenoid responsible for the reversal, in which case an audible sound should be heard between the cycles.