Causes
Remove the drain hose end from the nipple it's connected to below the sink or disposal, put the end in a bucket, then fill and drain the dishwasher to see if a garbage clog will simply blow out. If that doesn't work, but you see a small dribble of water, you may have to pull the dishwasher out and remove the drain hose in order clean it out or replace it. (Remember, there's still water in the unit—so put a drain pan under it before removing the hose.)
The check valve is located underneath the dishwasher, just before the drain hose. It’s a one-way valve, allowing water to drain but not return to the dishwasher. Remove the drain hose from the dishwasher (remembering to put a drain pan underneath) and check to make sure the valve is clean and opens correctly.