The 5 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 # 1489073
If the dishwasher won't drain, the pump and motor assembly might be defective. If the motor runs during the drain cycle, check for a blockage in the drain line. If the motor won't run during the drain cycle or if it just hums when it should be running, the pump and motor assembly might 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 # 2731
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 # 1193004
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.
RepairClinic Item # 523088
If the dishwasher won't drain the check ball might be stuck. The check ball is a small plastic ball in the check valve which prevents water from returning to the dishwasher after it has drained. If the ball gets stuck, it will prevent the water from draining also. The check valve is the device that the drain hose is attached to on the dishwasher.