Three places water can stop — and only one of them is a cleaning problem
A dishwasher smells because something organic is sitting wet somewhere it should not be, and the useful question is not «what do I pour in it» but which place is holding water. There are exactly three, and each one has its own smell, which is why your nose is the fastest diagnostic tool in the kitchen.
The filter. Modern machines have a manual-clean filter at the bottom — a coarse basket, then a fine mesh, then the sump below it. Unlike the old self-grinding models, this one catches food and keeps it, quietly, at body temperature, every time you run a cycle. Decomposing food smells sour, fishy or like a rubbish bin, and it smells worst while the machine is heating. Cleaning that filter resolves the large majority of smelly-dishwasher cases, takes five minutes and requires no parts at all — which is worth knowing before you buy anything.
The drain hose. If the smell is sewage, sulphur or rotten eggs, stop cleaning the inside of the machine. That smell is not coming from the tub, it is coming up the drain, and it is a plumbing fault: the drain hose has to rise to near counter height under the cabinet before it drops to the drain connection. That arc is what stops dirty water — and sewer gas — running back into the tub between cycles. Laid flat, or worse, running downhill from the machine, there is nothing to stop it. This is the single most important thing on this page: a sewage smell is not a hygiene problem and no amount of cleaning, vinegar or bicarbonate will change it, because the dirt is not in the machine.
The door seal. The rubber gasket around the door has folds, and the spray arms do not reach them. Water sits in the bottom corners, food and grease collect there, and the result is the musty, wet-basement smell that is strongest in the second you open the door and then fades. Run a finger along the gasket and look at it: black or brown residue means you have found it. The same three habits that keep a washing machine seal clean apply here — dry it, leave the door ajar, run something hot once a month.
The high loop, and the plug nobody removes
Two details explain almost every «it smells like a drain» case, and both are installation faults rather than machine faults. The first is the loop itself: the hose should climb to roughly 40 cm or more above the floor — in practice, as high as it will go under the cabinet — and then fall to the drain. If it lies along the cabinet floor or slopes downward the whole way, every pause between cycles is an open channel from the drain into your dishwasher. Securing it up high with a pipe clip or a zip tie is genuinely a five-minute fix, and it is the cheapest repair on this page.
The second is a parts-kit detail that catches out even careful installers: a new garbage disposal ships with its dishwasher inlet sealed by a plastic knockout plug, and that plug has to be knocked out and removed before the hose goes on. If the smell started the same week a disposal was replaced, that plug is the first thing to check, because it is the most commonly skipped step in the whole installation — and a machine whose drain is completely blocked will back up, smell, and look exactly like a machine that needs cleaning.
Both of those are worth checking before you spend anything. What is not worth doing is anything that treats the symptom: air fresheners, bowls of bicarbonate, scent beads. They work for an afternoon. And two things are actively bad: never pour strong alkali — washing powder, washing-up liquid, caustic soda — down the drain path, because it swells and destroys the rubber seals in the hose and pump, turning a smell into a leak; and never flush the sump with boiling water, because the corrugated plastic hose deforms, and the fat simply re-solidifies further down where it is harder to reach. If you want to clean the water path, citric acid at about 20 g per litre with hot water is the right tool, and it is what the machine's own descaling cycle is designed around.