When cleaning up after the holiday dinner, water backed up out of the disposal and into the dishwasher. It overflowed into the wood flooring and down to the level below. Is this a partial blockage in the disposal drain pipe that forced the drain water into the dishwasher? We have run the water and disposal since without issue. Is there a check valve in the 18-year-old Jenn-Air DU451 dishwasher that prevents such backups when working properly? The backup occurred twice in two days of heavy holidays use. Should we call a plumber or an appliance repairman?

If the dishwasher is installed correctly, the discharge hose should be risen above the drains of the sink before coming back down to the disposer inlet. This should prevent backflow into the dishwasher.

Perhaps if your hose was just "tacked up" someway, it could have fallen. Basically, it should back up in your sink before it backs up into the dishwasher.

That and it also sounds like you have a restriction in your drain lines.