Summary

Dishwashers drain by pumping wash water through a filter assembly, along a corrugated drain hose, and out through a non-return connection into the kitchen's waste system — either via a spigot on the sink trap or a dedicated standpipe. When water is left standing in the base of the machine at the end of a cycle, the fault is almost always somewhere in that path: something is physically blocking flow, the pump itself isn't running or isn't running with enough force, or the connection point is letting water siphon back in as fast as the machine can push it out.

The fault is one of the most common dishwasher callouts a plumber or appliance engineer sees, and the vast majority resolve without replacing the appliance. The filter and drain hose are user-serviceable and should always be checked first — no tools, no isolation needed beyond switching off. The drain pump and PCB sit behind that first-line check and need the machine isolated and partially disassembled.

This article covers the diagnostic tree from "standing water at the end of the cycle" through to pump replacement, the plumbing regulations that govern how the appliance is connected in the first place (because a badly installed standpipe or non-return valve causes symptoms identical to a blocked filter), and when to recommend appliance replacement over repair.

Key Facts

Quick Reference Table

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Symptom Most likely cause Fix
Water sits in base after full cycle, no pump noise on drain Pump motor failure or no power reaching pump Test pump for continuity/voltage; replace pump if dead
Water sits in base, pump hums/buzzes on drain Impeller jammed by foreign object Clear impeller of debris
Slow drainage, some water always left Filter partially blocked, hose kinked, or scale build-up Clean filter; check hose routing; descale
Machine drains then refills with dirty water Non-return valve stuck open, or waste connection height too low (siphoning) Replace valve; correct hose loop height
No drainage at all, filter and hose clear Blocked outlet spigot on sink trap, or downstream kitchen drain blocked Clear kitchen drain; check spigot knockout was removed at install
Drainage fine on short cycles, fails on long/heavy cycles Marginal pump wear, worsens under sustained load Replace pump proactively before full failure
E-code for drain fault displayed Refer to manufacturer code list first Brand-specific — narrows diagnosis before strip-down
Water drains but leaves standing puddle under sump cover Sump seal or gasket perished Replace sump seal

Detailed Guidance

Step 1: Isolate before opening the machine

  1. Switch off at the fused connection unit (FCU) or unplug — never work on an appliance with power connected.
  2. Turn off the appliance isolation valve on the cold feed (usually under the sink).
  3. Pull the machine forward carefully — most integrated dishwashers are screwed to the worktop or adjacent units; unscrew brackets before pulling.
  4. Have towels ready — standing water in the base will spill when the door is opened or the machine is tilted.

Step 2: Diagnostic tree

Dishwasher not draining
├── Check filter assembly first (always, every job)
│     ├── Blocked with food debris → clean, reassemble, test
│     └── Clear → proceed
│
├── Check drain hose
│     ├── Kinked or crushed behind machine → reroute, test
│     ├── Loop height too low / below spigot level → correct routing, re-secure
│     └── Clear and correctly routed → proceed
│
├── Run drain cycle and listen at pump
│     ├── Silent — no pump noise
│     │       → Check power reaches pump (multimeter at pump terminals during cycle)
│     │       → No voltage = PCB/wiring fault
│     │       → Voltage present, no rotation = pump motor failure → replace
│     │
│     └── Humming/buzzing — pump trying to run
│             → Impeller jammed → remove sump cover, clear obstruction
│             → Impeller clear but still won't drain → non-return valve stuck, or blocked outlet spigot
│
└── Water re-enters tub after draining
      → Non-return valve failed OR hose loop/connection height wrong
      → Correct installation before condemning the pump

Cleaning the filter assembly

  1. Remove the lower spray arm if it obstructs filter access (usually a quarter-turn or clip release).
  2. Twist and lift the coarse filter basket, then the fine micro-filter beneath it.
  3. Rinse both under a running tap; use an old toothbrush on the mesh to clear trapped debris.
  4. Check the sump beneath the filter for anything visible (glass fragments, bones, labels) before refitting.
  5. Refit in the correct order — the fine filter locates first, then the coarse basket locks over it. An incorrectly seated filter causes water to bypass filtration entirely and re-block the pump within a few cycles.

This takes 5–10 minutes and resolves the majority of "not draining" callouts on its own.

Clearing a blocked drain pump impeller

  1. With the machine isolated and filter assembly removed, locate the pump housing at the base of the tub (visible once the filter and sump cover are out).
  2. Some models allow impeller access directly from inside the tub via a removable cover; others require the machine to be laid on its back or side to access the pump from underneath.
  3. Rotate the impeller by hand — it should spin freely. Resistance or a grinding feel indicates trapped debris.
  4. Remove the obstruction with tweezers or long-nose pliers; do not force the impeller — pumps are often plastic and crack under excess torque.
  5. Reassemble, restore power and water, and run a test cycle before refitting the machine.

Replacing the drain pump

  1. Isolate power and water. Remove the machine from its housing.
  2. Access the pump from beneath — usually held by 1–2 spring clips or screws, with a hose clamp on the outlet side.
  3. Photograph wiring connections before disconnecting (drain pumps typically have a 2-pin connector).
  4. Release the hose clamp and pull the pump free.
  5. Fit the replacement, matched by model/part number — pumps are not universal even within the same brand.
  6. Reconnect wiring and hose, refit clamp, restore water and power.
  7. Run a full test cycle with the machine accessible (not yet pushed back in) to check for leaks at the hose joint before final refit.

Pump replacement typically costs £50–£110 in parts and 45–75 minutes labour, including refitting the machine.

Correcting a bad drain hose installation

The most common installation fault causing "drains then refills" symptoms is the drain hose not being looped high enough before it drops to the waste connection. Water needs to rise above the level of the connection point before falling away, or gravity will let it siphon straight back into the tub between pump cycles.

  1. Check the hose height at its highest point — should typically be at least 300mm above floor level, or clipped to the underside of the worktop with a purpose-made hose guide/clip.
  2. Confirm the non-return valve (either built into the machine's outlet or a separate in-line device) is fitted and not stuck.
  3. Where the dishwasher discharges via the sink's waste trap spigot, confirm the knockout plug inside the spigot was removed at first installation — a surprisingly common miss that causes complete drainage failure on a brand-new install.

Frequently Asked Questions

My customer's dishwasher smells bad and drains slowly — is this the same fault?

Usually a related but distinct issue: food debris trapped in the filter or sump is decomposing, which both restricts flow (slow drainage) and causes odour. Clean the filter and sump thoroughly, run an empty hot cycle with dishwasher cleaner, and check the drain hose for a low point where water and debris can pool and stagnate.

Can I just push the hose lower to increase drainage speed?

No — this is very likely to make things worse. A drain hose without a sufficiently high loop before the waste connection will siphon dirty water back into the tub between fills, which reads to the customer as "not draining" even though the pump is working fine. Follow the manufacturer's specified loop height, not intuition.

Is it worth repairing a dishwasher over 10 years old with a failed pump?

Usually yes if it's just the pump — parts are typically £50–£110 and the job is under 90 minutes. Only recommend replacement where the pump failure coincides with other ageing symptoms (door seal perished, PCB intermittent faults, heating element also failing) — replacing three or four components approaches the cost of a new appliance.

The dishwasher shares a waste connection with the kitchen sink — could the sink be the actual fault?

Yes, and this is worth checking before condemning any dishwasher part. If the kitchen sink itself drains slowly or gurgles, the shared waste run downstream of the dishwasher's spigot connection is likely blocked, and the dishwasher is draining into a restricted pipe. Clear the kitchen drain first — see kitchen waste and supply connections for connection layout.

What's the difference in diagnosing a dishwasher versus a washing machine drain fault?

The principle is identical — filter, hose, pump, non-return valve, in that order — but washing machines discharge far more water at higher force during a spin cycle, so the standpipe/air-gap detail matters more, and washing machines will refuse to spin at all if the drain cycle hasn't completed. See washing machine not spinning for the related mechanical fault.

Regulations & Standards