Dishwashers are meant to clean food residue from dishes, not to dispose of large bits of food stuck onto plates and pans. If you don't rinse your dishes before running them through your dishwasher, you may begin to notice an odor emitting from the drain, along with water backup and drain failure. Cleaning out the drain isn't a fun project, but it's one that in most cases you can easily complete without the help of a plumber.
Remove the bottom rack and set it aside. Reach into the back of the dishwasher and check the drain for any large pieces of food or debris. Clear any blockages.
Look under the sink and straighten the dishwasher drain hose if it has become twisted or kinked. Examine the point where the dishwasher's drain hose meets the garbage disposal. Remove the plastic disposer plug if it's still intact.
Check the back of the sink near the faucet for a plastic or metal air-gap valve. Open the dishwasher door a crack and remove the air-gap cover and cap. Remove any food or debris inside the air gap.
Pour 1 cup of vinegar into the dishwasher tub and run a cycle on the longest setting to remove odors and clean the dishwasher drain.