Why is it not plumbed via the disposal?
The drain pump uses an impeller to force water through the drain hose. If the drain pump motor is not working, the dishwasher won’t drain. To determine if the drain pump is defective, check the pump motor for continuity using a multimeter. If the motor does not have continuity, or if is getting power but won’t run, replace the drain pump. Drain pumps are not repairable—if the drain pump is defective, you must replace it.
The drain impeller pushes water through the drain hose and into the garbage disposer or drain line. If the impeller is broken, replace it.
Drain Solenoid Kit
The drain solenoid opens the drain valve to allow the water to exit the dishwasher. If the drain solenoid fails the valve will not open causing the dishwasher not to drain.