Switch the dishwasher off at the wall and turn off its water supply. You'll be reaching into standing water near an appliance on a mains circuit, and that's the one real hazard here. Wear gloves — broken glass and sharp debris are common in the filter, and the water will smell. Anything involving the machine's wiring, or plumbing beyond the accessible filter and hose, is a job for someone qualified. Your manual takes precedence over anything below.
Where the water is meant to go
The route out is short, and every part of it is mechanical.
The filter, which is usually it
At the bottom of the tub, under the lower spray arm, sits a filter assembly — usually a cylindrical mesh that twists out, sometimes with a flat coarse screen underneath it. Its job is to catch food debris.
Modern machines mostly use a filter you're expected to clean rather than the older self-cleaning grinder design. That change is why this problem became so common: the maintenance step exists, and almost nobody knows about it.
What to do and what to expect:
- Bail the standing water out first with a cup and a sponge. Working in it is unpleasant and you can't see what you're doing.
- Twist the filter anticlockwise and lift it out. It may be stiff if it's never been removed.
- Rinse it under a hot tap with a soft brush. Grease and starch build a film that blocks the mesh even when it looks clean.
- Look in the recess underneath. Glass fragments, olive stones, labels off jars, bits of bone — this is where they sit. Feel around carefully with gloves on.
- Refit it properly. A filter that isn't seated and locked lets debris straight into the pump, which turns a small problem into a real one.
The disposal plug — if it never drained
This one deserves its own section because it's specific, extremely common, and invisible unless you know it exists.
Where a dishwasher drains into a waste disposer, the hose connects to a spigot on the side of the disposer unit. Disposers ship from the factory with that spigot sealed by a moulded knockout plug, because not every installation uses it. Whoever fits the disposer is supposed to punch that plug out before connecting the dishwasher hose.
If they don't, the dishwasher pumps against a solid wall. The machine is fine. The hose is fine. The water simply has nowhere to go.
The signature is unmistakable once you know it:
- The dishwasher has never drained properly since a new disposer was installed, or since a kitchen was fitted.
- It was working, a disposer was replaced, and it stopped that same day.
- A brand-new dishwasher has never drained once.
If any of those match, the plug is the first thing to check — before touching the filter. It's a documented step in every disposer's installation instructions, and it's skipped constantly.
A related cause in the same place: the disposer itself is full. A disposer packed with waste blocks the dishwasher's outlet. Running it for a few seconds with the cold tap on is free and occasionally solves the whole thing.
The hose, the high loop and the air gap
Three separate things live between the pump and the drain, and each fails differently.
The hose can kink. Very common after a machine has been pulled out and pushed back for any reason — the hose gets trapped against the wall or under a foot. Free to fix and easy to miss.
The hose can block internally with grease and starch, which build a soft sludge over years. Detaching it into a bucket and flushing it settles this quickly.
The high loop matters more than it sounds. The drain hose is meant to rise up to just under the worktop before dropping to the waste connection. That loop stops dirty sink water siphoning back into the dishwasher. If it's been pulled down flat during an installation, you get standing water and often a smell — and no amount of cleaning fixes it, because the machine drains and then refills itself from the sink.
The air gap — the small chrome cylinder on the worktop next to the tap, common in some regions and required by code in others — does the same job differently. It blocks with debris, and when it does, water backs up. Its cap lifts off and the inside can be cleaned out. A visible sign is water spitting out of the air gap onto the worktop during a cycle.
When it really is the pump
Pumps do fail, just far less often than people assume. What distinguishes it:
- Silence at the drain stage. A working drain pump makes a distinct whirring hum for around a minute. Nothing audible at all points at the pump, its wiring, or the control board.
- A grinding or rattling when it tries — usually something jammed against the impeller rather than a dead motor. Often clearable.
- It hums but nothing moves. A seized impeller.
- The filter is genuinely clear, the hose is clear, and the disposal path is open — and it still won't empty.
One more possibility worth knowing: the check valve. Many machines have a small one-way valve after the pump that stops drained water flowing back. When it sticks shut, the machine can't push water out; when it sticks open, water returns into the tub after a successful drain. The second symptom — drains fine, then refills — is characteristic.
Why the dishes came out dirty too
People report these as two problems. They're usually one.
A dishwasher recirculates its wash water through the spray arms during the cycle. If the filter is clogged, the water going to the spray arms is dirty and restricted — so the machine washes your dishes in food debris and at lower pressure. Then it can't drain.
So a clogged filter produces gritty dishes, standing water, and a smell, all at once. Clearing it fixes all three, which is why it's worth doing before assuming anything is broken.
Two related checks while you're in there: spin the spray arms by hand to confirm they turn freely, and hold them up to the light to see whether the small holes are blocked with limescale or debris. Blocked jets produce patchy cleaning that looks like a detergent problem.
What to check, in order
- Power off at the wall, water off.
- Did it ever drain properly? If a disposer or kitchen was recently fitted and it has never worked — go straight to the knockout plug.
- Run the waste disposer for a few seconds with cold water. Free.
- Check the drain hose isn't kinked behind or under the machine.
- Check whether the kitchen sink drains normally. If it doesn't, the problem is your plumbing, not the appliance.
- Bail out the water, remove and clean the filter, and feel in the recess beneath it.
- Clean the air gap if you have one on the worktop.
- Refit, restore power, run a short cycle and listen at the drain stage.
- Still nothing? Detach the hose into a bucket and run a drain cycle. Water under pressure means the machine is pumping and the blockage is downstream. No water means it's the pump or upstream.
Stopping it happening again
- Clean the filter monthly. Two minutes. This single habit prevents most of these failures.
- Scrape plates, don't rinse them. Scraping removes the bones, stones and labels that block filters. Heavy pre-rinsing is unnecessary on modern machines and can actually confuse the soil sensor into running a lighter wash.
- Run a hot maintenance cycle occasionally, empty, to clear grease from the hose and sump.
- Check the hose after anyone moves the machine. The most common cause of a dishwasher that stopped draining the day after a kitchen was rearranged.
- Don't overdose detergent. Residue builds in the hose and contributes to slow draining.
- If a disposer is ever replaced, confirm the knockout plug was removed. Ask, specifically.
Standing water means the path blocked, not that the pump died — and two of the five blockage points are under your sink rather than in the machine. If it has never drained since a new waste disposer or kitchen was fitted, check the disposer's knockout plug first: it ships sealed and must be punched out, and skipping that step is one of the most common causes there is. Otherwise it's the filter, which modern machines expect you to clean and almost nobody does — and a clogged filter explains the dirty dishes and the smell at the same time. A kinked hose behind the machine and a slow kitchen sink are the two other free things to rule out before anyone charges you for a visit.