Dishwasher Won't Drain: Causes, Error Codes & Fixes
A dishwasher with water standing in the bottom is a drain-chain problem with four links: the filter, the pump, the hose, and the sink connection (air gap or disposal). Work them in order — the filter alone is the answer often enough that skipping it is the classic waste of an afternoon.
The filter and the sump
Remove the filter assembly and wash it properly (hot soapy water, soft brush). A filter can look 'okay' and still be grease-blinded. While it's out, inspect the sump for glass, seeds and label debris — and on Bosch machines, check the pump cover is present and clicked in.
The sink end: air gap and disposal
If you have an air gap on the sink, pop the cap and clear the inner tube — it plugs silently. New garbage disposal + sudden no-drain = the disposal's dishwasher knock-out plug was never removed; it's a 10-minute fix that saves a service call.
The pump's testimony
Run a cancel/drain and listen: a healthy hum with no movement = jam downstream or in the impeller; buzzing = jammed impeller; silence = dead/unpowered pump. Unplug before reaching into the pump area — glass lives there.
Codes that mean 'won't drain'
Bosch E24/E25, Samsung 5C/SC/OE, LG OE-family, GE FTD, Frigidaire i20/E20 — your brand page has the model-specific filter and pump locations.
The high loop and the air gap: plumbing that drains uphill
A dishwasher drain hose does not simply run downhill to the sink, and the deliberate uphill section is where many no-drain faults live. Installations use one of two backflow guards: a high loop, where the hose is fastened up under the countertop before dropping to the drain connection, or a countertop air gap, the small capped cylinder beside the faucet on many sinks. Bosch's installation guidance tells the air-gap story plainly: water is pumped up through the gap and down the larger hose to the drain, and small pieces of food lodge between the plastic lid and the inner tube until the trickle becomes a blockage — a surprisingly small amount of debris doing a surprisingly complete job. The owner-level service follows from the design: pop the air gap's cap, lift out the inner piece, and clear the tube; on a high-loop installation, check instead that the loop is still fastened up where it belongs, because a loop that has fallen off its mounting lets sink water siphon back into the tub — the 'it drains, then dirty water returns' complaint — and a hose that sags into a low belly collects its own sediment trap. Neither check needs the machine moved, and both sit downstream of the pump, which is why they come before any pump conclusion: a healthy pump pushing against a blocked gap presents exactly like a dead pump, at a fraction of the repair cost.
The disposal connection: knockout plugs and shared drains
Where the dishwasher drains through a garbage disposal, the disposal is part of the dishwasher's drain chain whether it wants to be or not. The classic installation fault is the knockout plug: disposals ship with a solid plug inside the dishwasher inlet nipple, the installer is meant to punch it out when connecting the hose, and a missed plug means the dishwasher has been unable to drain since the day the disposal went in — which is why 'stopped draining the week the disposal was replaced' is a diagnosis, not a coincidence. The plug is checked with the disposal's power off and the hose removed, and the punched-out disc must be retrieved from inside the unit rather than left to wander. The everyday version of the same link is simpler: the dishwasher and the disposal share the drain, and a disposal chamber full of ground food is a partial blockage the pump must push through. Running the disposal with cold water before a dishwasher cycle clears that chamber, and a sink that itself drains slowly is telling you the shared line — not the dishwasher — is the restriction: no pump in the dishwasher can out-drain a sink drain that will not take water from a tap. Work the sink end first whenever the timeline mentions plumbing work, a slow sink, or a new disposal; it is the only link in the chain you can fully test with a kettle of water and thirty seconds of attention.
Hose autopsy: kinks, sags and the bucket test
Between the pump and the sink, the drain hose fails in three physical ways, all visible. It kinks — classically where the machine was pushed back into its opening and the hose folded against the cabinet or the wall, a fault that dates precisely from an installation or a repair and worsens as the fold relaxes shut. It sags — an unsecured span bellies down, fills with greasy water that cools and coats, and narrows from inside over months rather than days. And it plugs at its narrow points, the barbed fittings at either end, where a single cherry stone, shard or label skin can cork the whole run. The decisive experiment is the bucket test, and it splits the machine from the plumbing in one cycle: disconnect the hose at the sink end (power off at the breaker while you work under there, water ready to spill), direct the hose into a bucket, and run a cancel/drain. A strong stream into the bucket acquits the pump and the machine-side hose and convicts the sink plumbing; a feeble trickle convicts everything upstream of the bucket — filter, pump, or the hose itself — in that order. A hose that fails inspection is replaced, not cleaned back to life: corrugated drain hose is inexpensive, its internal ridges are the sediment's home, and refitting a new hose with the high loop restored and no sags is the permanent version of the fix.
Filters by brand: same job, different furniture
Every dishwasher filters its sump water, but the furniture differs enough that owners routinely clean the wrong thing or miss the second stage. Bosch machines use a cylindrical filter that twists out of the sump floor, often paired with a flat mesh plate, and their drain codes assume both are clean — and the small pump cover beside them must be present and clicked home, a detail Bosch's own E25 guidance turns on, because a missing cover lets debris reach the impeller the filter was protecting. Samsung and LG use multi-part filter assemblies that separate into coarse and fine stages, and the fine stage is the one that grease-blinds invisibly: it can look merely dull while passing almost nothing, which is why washing — hot water, detergent, a soft brush, and a rinse until water runs through the mesh freely — is the standard for all of them, never a shake over the bin. GE and Frigidaire designs vary between fine filters and (on some models) a glass trap and a coarse screen under the lower spray arm. The universal rules survive the variations: the filter comes out before any other drain diagnosis, it goes back in correctly seated and locked (a filter left unlatched lets debris bypass straight to the pump it guards), and the sump it reveals gets inspected while access is free — glass, bones, seeds, and the occasional twist tie live down there, exactly where the pump intake can find them.
Reading the pump like a witness
The drain pump narrates its own condition if you give it a quiet room and a cancel/drain cycle, and its testimony narrows the fault before a single panel moves. A healthy hum followed by the sound of moving water that then stops is a pump doing its job against a downstream blockage — the investigation belongs to the hose, gap and disposal. A hum with no water movement at all localises the problem to the pump's immediate neighbourhood: something in the impeller, or a collapsed hose right at the outlet. A buzz that is louder and angrier than a hum is an impeller that cannot turn — a jam, classically glass or a seed — and the machine should be switched off rather than left buzzing, because a stalled pump motor heats while it strains. Silence, with power confirmed at the machine, is the electrical branch: the pump is not being fed or has failed open, and that is multimeter territory and usually a service visit. Two safety notes govern all of this, and they are not boilerplate: the power goes off before any hand enters the sump or pump area, because broken glass collects there invisibly; and standing water that has sat for days should be bailed out (cup, then sponge, then towel) before work begins, both for the smell and because a tub you can see into is a tub you can actually inspect. The brand code pages on this site — Bosch E24/E25, Samsung 5C/SC, LG OE, GE FTD, Frigidaire i20 — pick the story up at exactly the point the pump's testimony ends.
Keeping it drained: the habits that prevent the sequel
A dishwasher that has drained badly once will do it again on the same diet, and the prevention list is short because the failure list is short. Scrape plates into the bin rather than rinsing them into the machine: the filter's workload, the sump's debris load and the hose's sediment all begin at what leaves the plate. Run the filter wash on a rhythm matched to use — weekly for a machine that runs daily, monthly at lightest use — and treat any loading of bone-in, seeded or glass-risk items as a prompt to check the sump afterwards, because those are the objects the pump later meets. Keep the disposal run and the sink draining freely if they share the line, and re-check the air gap's capful of debris whenever dishes start coming out gritty, which is the gap's early warning. Use the machine's heat and detergent properly: chronic cool, short eco cycles on greasy loads lay down the very film that blinds fine filters, and an occasional full-temperature cycle with a proper detergent dose is maintenance, not waste. Finally, respect the timeline evidence this page began with — a no-drain that starts the day after plumbing work, a machine move, or a repair is an installation fault until proven otherwise (knockout plug, kinked hose, fallen loop), while one that creeps in over months is a maintenance fault. Naming which kind you have, before touching anything, is most of the diagnosis — and it is free.
How much standing water is normal?
Before any diagnosis, calibrate the complaint: a dishwasher that has drained correctly does not leave a dry sump. A shallow film of water over the filter area — and, on many models, water visible in the sump below the filter when the filter is removed — is the designed state: it keeps pump seals wet and, in designs with a trap, blocks drain odours from rising into the tub. What is not normal is water standing above the filter, covering the tub floor, or deep enough to lap at the door seal when the door opens; that is the failed drain this page addresses. The timing test sharpens it further: water present immediately after a cycle that then slowly disappears over hours is a drain that is partially restricted or back-flowing (the hose, gap and check-valve territory), while water that never moves at all between cycles is a pump-or-total-blockage event. Smell is a soft witness too — standing water that smells of the sink suggests backflow from the shared drain, while water that smells of the last wash suggests it simply never left. Owners not infrequently 'fix' a normal sump with a service call; equally, a genuinely failed drain gets tolerated for weeks because 'there is always some water in it'. The distinction — above the filter line, or below it — is the whole triage, and it takes five seconds at the open door.
The check valve and the mystery of returning water
One small part explains the most confusing drain complaint of all: the dishwasher empties, the cycle ends dry — and an hour later there is water in the tub again. Inside or just beyond the pump outlet sits a check valve, a simple flap whose only job is to let water out and refuse its return. When the flap sticks open, warps, or is held ajar by a seed or a scale flake, the column of dirty water standing in the drain hose — everything between the machine and the high loop's crest — drains back into the sump once the pump stops. The volume matches the complaint exactly: not a tub full, but a sump-and-filter-covering return that looks like a fresh failure. The same return path, on a larger scale, is what a fallen high loop or a blocked air gap produces from the sink side, so the three checks travel together: valve flap free and seating, loop still high, gap clear. The valve itself is an inexpensive, defined part on most models, accessed from the sump or the pump outlet with the power off; a flap that is merely fouled can be cleaned and reseated, while a warped or torn one is replaced. Diagnose it by its schedule — clean machine at cycle end, water back within the hour, no new water added — because that timetable belongs to no other fault in the chain, and it acquits the pump completely.
Key takeaways
- Scrape plates before loading: the drain chain's workload is set at the sink, not in the machine.
- Standing water left for days grows the smell owners blame on the machine — drain it manually (cup + sponge) while you arrange the fix.