What is an air gap?

An air gap is a physical break in the drain line, mounted through the countertop, that makes backflow into the dishwasher impossible. It is required by code in some areas and optional in others. Because everything the dishwasher drains passes through it, it collects debris and eventually overflows onto the counter or blocks the drain entirely — a common cause of a "not draining" call where the machine itself is fine.