The 4 most common part(s) or condition(s) which cause the symptom Dishwasher won't drain are listed below starting with the most likely. Check or test each item and watch any available videos.
RepairClinic Item # 1387293
If the dishwasher won't drain, the drain pump might be defective. Test the pump motor with an Ohm meter. Drain pumps are not repairable, if it's defective it will need to be replaced.
If the dishwasher won't drain and the garbage disposer was installed recently, the installer might have forgotten to knock out the drain plug from this disposer. All garbage disposers are sold with a solid plug in the drain tube for situations where a dishwasher isn't connected. However, if the dishwasher is connected, the plug will need to be removed. If the disposer wasn't replaced recently, remove the dishwasher drain hose from the disposer and clean out both the fitting on the disposer and the end of the drain hose. These are the most common places to find an obstruction if the dishwasher won't drain.
RepairClinic Item # 473645
If the dishwasher won't drain the drain hose might be clogged. Remove the drain hose from both the dishwasher and the sink drain to clean it out. The most common place for an obstruction is at the garbage disposer.
RepairClinic Item # 935361
If the dishwasher won't drain the check valve might be stuck closed. The check valve is a device which prevents water from returning to the dishwasher after it has drained. If the valve gets stuck closed, it will prevent the water from draining. The check valve is located in or at the outlet of the pump that the drain hose attaches to. Sometimes the check valve is accessed from the inside of the unit depending on the model.