The first thing I would check is the air gap (if any) on top of the sink.
Remove the cover and the top piece and check for clog.
If there is nothing wrong then the problem is a clogged check valve (#15 on the
pump & motor break down diagram).
Remove all water from the dishwasher.
Remove the bottom panel.
Remove the drain hose from the check valve (be prepared for some water on the floor).
Unscrew the check valve and make sure it's clean.
Here are the break down diagrams for the Whirlpool dishwasher
Model DU920PWKQ0
Gene.