My dishwasher leaves debris behind. As an experiment, I left a clean coffee mug in the bottom of my dishwasher then ran it empty for a full cycle. When the machine stopped, there was a lot of stuff in the mug that I can only describe as wet bread crumbs and a black sandlike substance. Can anyone help? Thanks.

Check to see if the strainer in the bottom of the DW is clean. Sometimes they plug up and cause problems. Other times it is because the pump is not removing all the water from the tub before the rinse cycle.

The best way to stop this happening is to rinse pots and plates of all food residue before putting them into the dishwasher. The grit and sand you refer to is usually broken down food particles. Use a good dishwasher cleaner to get rid of the debris and rinse before use.
Guinness683

I never rinse my dishes before using the dishwasher. I do remove the chicken bones etc. Dishwashers are equipped with a soft food disposer built in. If you are going to rinse the dishes first you might as well put some soap in the water and wash them while you are at it. This will save the cost of operating the dish washer.

I have been repairing dishwashers for over forty years, so I guess you know best.
Guinness683

I have been repairing many items since 1958 and I also owned a Whirlpool Teck Care Center/Dealer also. The manufactures use and care manual does not tell you to clean the dishes before you put them into the dishwasher. Now there is nothing wrong with precleaning but it just wastes the homeowners valuable time. Most problems of this sort are plugged or restricted drain or air gap, clogged drain holes in the dishwasher screen below the lower wash arm,cheap powder dishwasher detergent and a weak pump assembly. The water level can also cause problems. To low a level will not allow proper water circulation during the wash.