The pattern is so reliable we can diagnose it on the phone: “We replaced the garbage disposal on Saturday, and now the dishwasher holds water.” Congratulations — you've met the knockout plug, the most profitable little disc of plastic in the appliance business.
What the knockout plug is
Every new disposal ships with its dishwasher inlet sealed by a molded plug, because plenty of sinks have no dishwasher at all. If a dishwasher drain hose connects there, the plug must be punched out with a screwdriver and hammer before the hose goes on. Skip that step and the dishwasher pumps its wash water against a dead end, then quits with a full sump. The machine is perfectly healthy; its exit is bricked shut.
How we confirm it
Timing is the tell — a drain failure the same week as a disposal swap is the plug until proven otherwise. We disconnect the hose at the disposal inlet and check for the plug; if it's there, it gets punched out, fished from inside the disposal chamber (leave the shards in and they'll rattle forever), and the hose reclamped. Ten minutes, done.
While we're under the sink
Disposal swaps disturb the whole drain neighborhood, so we check the rest: the drain hose's high loop or air gap, which keeps sink water from siphoning back into the dishwasher — installers in a hurry let the hose sag, and a low hose means dirty water and swampy smells later. We also verify the hose isn't kinked behind the cabinet and that the clamp actually bites. Half the “mystery” dishwasher smells in renovated kitchens trace back to this one under-sink geometry.
If there was no disposal swap
A drain failure with no plumbing work in the story points elsewhere: chopped food in the drain pump, a clogged filter assembly, or a check valve stuck shut. Different fix, same format — we open the path end to end, run a full drain test, and hand you one written price before the work. Either way, standing water sours fast in a warm kitchen; sooner beats later.