You open the dishwasher expecting clean plates and get hit with a stale, sour smell instead. If you are wondering why the dishwasher smells bad, the answer is usually simple – something inside is holding dirty water, food residue or grease for longer than it should.
The good news is that a bad-smelling dishwasher does not always mean an expensive fault. In many cases, the cause is a blocked filter, debris in the spray arms, a drain issue or a habit that lets moisture sit in the machine for too long. The less good news is that smells tend to get worse when they are ignored, and sometimes they point to a drainage or heating problem that needs proper diagnosis.
Why the dishwasher smells bad in the first place
A dishwasher is designed to wash away food and detergent, but it still works in a damp, enclosed space. That means even a small amount of trapped waste can turn into an odour very quickly. Grease coats internal parts, bits of food settle into filters and drains, and standing water becomes the perfect place for bacteria to build up.
Smells are often strongest after a cycle has finished and the door has stayed shut for hours. The warmth and moisture remain trapped inside, which gives any residue more time to produce a musty or rotten smell. If the odour smells like drains or sewage rather than stale food, that usually points to a drainage issue rather than a cleaning problem.
The most common causes of a smelly dishwasher
A dirty filter
This is the first place to check. The filter sits at the bottom of the tub and catches food particles so they do not block the pump. Over time, it can become coated with soft food, grease and detergent sludge. Once that build-up starts to break down, the smell becomes obvious.
A filter problem is especially common in busy households, rental properties and office kitchens where the machine is loaded often and nobody checks the base for debris. Even if plates look clean at the end of the cycle, the smell can still be coming from what is trapped below.
Standing water in the base
A dishwasher should drain almost completely. If you notice water sitting at the bottom after a cycle, that is a warning sign. It may be caused by a partial blockage in the filter area, the waste hose, the sink spigot or the pump.
This kind of smell is usually heavier and more unpleasant because dirty water is being left behind between washes. If it has a drain-like smell, the plumbing connection may be allowing odours to come back into the appliance.
Grease and detergent residue
Not every smell comes from visible food waste. Grease can build up along the door seal, inside the lower edge of the door, around the hinges and in the spray arms. Detergent and rinse aid can also leave a film if the wrong amount is being used or if cooler cycles are relied on all the time.
Eco programmes can be useful for energy saving, but they do not always shift heavy grease if the machine is already dirty inside. That is where a dishwasher can start smelling even though it still appears to be washing properly.
Mould on seals and hidden edges
The rubber door seal often gets missed during routine cleaning. Moisture sits there after every cycle, and if the door stays closed all day the seal can become slimy or mouldy. You may also find build-up in corners, under the lip of the door or around the detergent drawer.
This tends to cause a musty smell rather than a strong drain odour. It is common in machines that are used infrequently or left closed for long periods.
Blocked spray arms
Spray arms can collect limescale, food particles and grease. When that happens, water flow is reduced and dishes may not rinse as well as they should. The result is more residue left inside the machine, which then contributes to smells.
This problem is easy to overlook because the spray arms are not always checked unless cleaning performance drops noticeably.
How to fix a dishwasher that smells bad
Start with the simplest checks before assuming there is a mechanical fault. In many cases, a proper clean and a drainage inspection solve the issue.
First, remove the bottom rack and take out the filter. Wash it in warm water with washing-up liquid and a soft brush. If there is debris in the filter housing, clear that as well. Do not force anything sharp into the pump area, as that can damage components.
Next, inspect the base for standing water. A small amount can be normal on some models, but pooled dirty water is not. If water is sitting there after a cycle, the machine may not be draining correctly.
Wipe the door seals, the lower inside edge of the door and the corners where grime collects. These areas are often responsible for smells that linger even after the main tub has been cleaned.
After that, run a hot empty cycle with a dishwasher cleaner suitable for your appliance. A maintenance wash helps break down grease and residue in places you cannot easily reach. White vinegar is often suggested, but it is not ideal for every machine or every seal, so a proper cleaner is usually the safer option.
If the spray arms are removable on your model, take them out and rinse through the holes. If they are blocked, water pressure drops and cleaning results suffer.
When the smell is really a drainage fault
If you clean the machine thoroughly and the smell returns quickly, look closer at how it drains. A kinked or partially blocked waste hose can hold dirty water. A poor plumbing connection under the sink can also send drain odours back towards the dishwasher.
This is particularly common after a new installation, a kitchen refit or DIY work under the sink. If the waste hose has not been looped correctly, or the spigot connection is restricted, smells can start almost immediately.
A failing drain pump can cause similar symptoms. You may hear unusual noises, notice water left in the tub or find that cycles seem to finish without fully clearing the machine. At that point, it makes sense to have the appliance checked rather than continuing with repeated cleaning products.
Why the smell sometimes means poor heating
A less obvious cause is poor water heating. Dishwashers rely on the right temperature to dissolve detergent properly and break down grease. If the water is not getting hot enough because of a faulty heater, thermostat or control issue, the machine may leave behind more residue than it should.
This is one of those faults that can be missed because the dishwasher still runs and may even leave dishes looking mostly clean. But over time, the inside develops a stale smell, glasses may come out cloudy and grease starts to build up around hidden components.
If you have already cleaned the filter and checked the drain, but the smell keeps returning alongside poor wash results, a heating fault is worth considering.
How to stop the smell coming back
Prevention matters more than people think. You do not need to rinse every plate perfectly before loading, but scraping off heavy food makes a real difference. Clean the filter regularly, especially if the dishwasher is used daily. Run a hot maintenance cycle from time to time rather than relying only on low-temperature programmes.
It also helps to leave the door slightly ajar after a wash when practical and safe to do so. That lets moisture escape instead of sitting on the seals and internal surfaces.
If you manage a rental property or a busy household, simple routine checks can prevent avoidable call-outs. A dishwasher that smells bad is often giving early warning before it turns into a blockage, pump issue or leak risk.
When to call an engineer
If the odour is strong, returns quickly after cleaning, or comes with standing water, poor draining, unusual noises or weak cleaning results, it is time for proper diagnosis. The same applies if the smell is sewage-like, because that can point to a drainage connection problem rather than a hygiene issue.
A qualified appliance engineer can check the pump, hose routing, filters, heating system and internal components without guesswork. That is usually faster and more cost-effective than trying one cleaning trick after another while the fault gets worse.
For London households, landlords and property managers, speed matters. A dishwasher out of action can disrupt the whole kitchen, and in shared or tenanted properties smells tend to become a bigger complaint than the original fault.
At FaultFree Engineering Group, we see this regularly: what starts as an unpleasant smell often turns out to be a blocked drain path, a pump restriction or a heating issue that needs a proper repair, not another bottle of cleaner.
A dishwasher should smell neutral, not swampy, sour or like the drain. If it does not, trust the warning sign and deal with it early – it is usually easier, cleaner and cheaper that way.