Help...Dishwasher Won't Drain

Could I get some informed guesses on a situation?
The patient is a Sears-rebranded Whirpool of about 5 years vintage. Pretty standard - conforms to a number of Whirlpool diagrams I see on the web. A circular finely-screened cover under the lower pump arm is held on via 8 Torx screws, and upon being removed shows the rotating pump.
The unit suddenly stopped draining. On a drain cycle, there's lackluster pump action for a while, and then just a spurt of water every second or so. The result is that about half the water is pumped out, and half still remains in the unit.
The hose between pump and airgap is clear.
Upon removing the hose, and starting the fill cycle, water immediately drains through the pump drain port via gravity.
On the drain cycle, there's evidence that water is being pumped through the spray arms rather than (totally) through the drain port, because concave utensils on the top rack fill with water even after being manually emptied as an experiment.
Could this be the mythical check valve? I would think that valve would sit between the drain port and the hose to prevent/cause water go go through the port rather than through the spray arms, but obviously I'm mistaken. Where is this check valve, and how does it do its thing?
Advice eagerly solicited - after two days of pumping out the unit with my Rainbow vac, it's getting to be annoying!
Art
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Should be a 2 way valve actuated by a solenoid which directs the pump output to either the drain or the spray arm. Sounds like it is stuck in the middle or on recirculate. Could be mechanically jammed or lost power to the solenoid or the flap is physically broken.
Most likely a piece of broken glass, melted plastic spoon, etc. jamming the valve's movement.
Short of disassembling anything, Try forcing water backward through the drain port to flush debris.

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There are two types of dishwashers. One design uses a single pump to wash and drain. I believe they use a solenoid to switch from one to the other. The other design uses two pumps, one for wash, one for drain. Guess you have to figure out which you have and then go from there. There are plenty of parts blow out diagrams at appliance parts online.
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