The short answer
On an older Cosmo dishwasher, a drain failure ranks differently: the filter has been cleaned many times but the sump and the hose have not, grease has narrowed the drain path over years, and the check valve has had a long life. Start at the filter as always, but expect the answer to be accumulated grease or a tired pump rather than an installation error.
What you are actually seeing
- Draining has become slower over months rather than failing suddenly — Means: on-page
- The pool at the end of a cycle has grown gradually deeper — Means: on-page
- The drain sounds strained or labours before the water clears — Means: on-page
- Cleaning the filter helps for a few cycles and then the problem returns — Means: on-page
- A film of grease is visible in the sump and around the filter housing — Means: on-page
- The cycle stops early with a full tub and a code — Means: different-fault — Go to: cosmo-dishwasher-cycle-fault
- Water on the floor rather than in the tub — Means: different-fault — Go to: cosmo-dishwasher-water-leak
- Body: Match what you see before you take anything apart. Draining has become slower over months rather than failing suddenly. The pool at the end of a cycle has grown gradually deeper. The drain sounds strained or labours before the water clears. Cleaning the filter helps for a few cycles and then the problem returns. A film of grease is visible in the sump and around the filter housing. Some near-miss symptoms belong on a different page: The cycle stops early with a full tub and a code points at a cycle or control fault, covered on cosmo-dishwasher-cycle-fault; Water on the floor rather than in the tub points at a leak, covered on cosmo-dishwasher-water-leak.
Causes, ranked for this machine
- Grease and scale narrowing the drain hose and the sump over years — Why this rank: First on an older machine because the loss is gradual, which matches a symptom that has crept up rather than appeared overnight
- A drain pump that is losing performance rather than failing outright — Why this rank: A labouring drain that eventually clears is a wearing pump, and it will not show as a fault code
- A check valve holding partly open and letting water back into the tub — Why this rank: Explains a tub that drains and then partly refills, which owners read as a failure to drain at all
- A drain hose that has sagged over years as the cabinet has been used — Why this rank: Long service means the hose has been moved many times, and each time it is a little lower
- A blocked filter or air gap — Why this rank: Still worth clearing first because it is free, but it ranks lower on a machine whose owner already cleans the filter regularly
- Body: Ranked for a built-in kitchen dishwasher with an electronic control, a manual filter and a drain hose running to a disposer or an air gap. Most likely: Grease and scale narrowing the drain hose and the sump over years. First on an older machine because the loss is gradual, which matches a symptom that has crept up rather than appeared overnight. Next most likely: A drain pump that is losing performance rather than failing outright. A labouring drain that eventually clears is a wearing pump, and it will not show as a fault code. Third: A check valve holding partly open and letting water back into the tub. Explains a tub that drains and then partly refills, which owners read as a failure to drain at all. Fourth: A drain hose that has sagged over years as the cabinet has been used. Long service means the hose has been moved many times, and each time it is a little lower. Fifth: A blocked filter or air gap. Still worth clearing first because it is free, but it ranks lower on a machine whose owner already cleans the filter regularly.
What to check, in order
- Clean the filter and sump first, then judge whether the improvement lasts more than a few cycles
- Run a hot cycle with a dishwasher cleaner to strip grease from the hose and sump
- Listen to the drain and note whether it labours or runs freely
- Check whether the tub partly refills after draining, which points at the check valve
- Body: Each check rules out the causes above it. First, clean the filter and sump first, then judge whether the improvement lasts more than a few cycles. Second, run a hot cycle with a dishwasher cleaner to strip grease from the hose and sump. Third, listen to the drain and note whether it labours or runs freely. Fourth, check whether the tub partly refills after draining, which points at the check valve.
Checks you can safely make yourself
- Switch off at the breaker, remove the filter, clean it and wipe the grease film from the sump and the filter housing — Tools: cloth, brush, gloves — Time estimate: half an hour
- Run the hottest cycle with a proprietary dishwasher cleaner and an empty machine, then check whether draining improves — Tools: dishwasher cleaner — Time estimate: one long cycle
- Listen at the machine during the drain and note whether the pump labours, surges or runs smoothly — Tools: notepad — Time estimate: one cycle
- Drain the machine, wait, and check whether water has returned to the sump, which points at the check valve rather than the pump — Tools: torch — Time estimate: half an hour
- Check the drain hose height under the sink and restore a high loop if it has sagged over the years — Tools: cable tie — Time estimate: a quarter of an hour
- Body: Checks an owner can make with the machine switched off at the breaker, or by watching a cycle. Step one: Switch off at the breaker, remove the filter, clean it and wipe the grease film from the sump and the filter housing. Tools: cloth, brush, gloves. Time: half an hour. Step two: Run the hottest cycle with a proprietary dishwasher cleaner and an empty machine, then check whether draining improves. Tools: dishwasher cleaner. Time: one long cycle. Step three: Listen at the machine during the drain and note whether the pump labours, surges or runs smoothly. Tools: notepad. Time: one cycle. Step four: Drain the machine, wait, and check whether water has returned to the sump, which points at the check valve rather than the pump. Tools: torch. Time: half an hour. Step five: Check the drain hose height under the sink and restore a high loop if it has sagged over the years. Tools: cable tie. Time: a quarter of an hour.
Safety first
Safety
Switch off at the breaker before bailing the tub or reaching into the sump, and wear gloves: broken glass in the sump is the most common injury on this job. Never run the disposer with a hand near it. On a machine of this age, treat any connection under the sink as suspect before you touch it: corrosion and hardened insulation are both cumulative, and neither is visible until a panel comes off. Unplug the appliance or switch off its breaker before removing any panel. A capacitor can hold a charge after the power is off. unplug-before-service
Where the owner fix ends
On an older machine, call a technician once the filter and sump are clean, a cleaner cycle has run and draining is still slow or still labours. A worn drain pump or a failed check valve needs the machine pulled and the lower panel removed, and on an aged machine that is the moment to weigh the repair against replacement.
Nearest pages of the same kind
Generalized information
This page is generalized across the Cosmo dishwasher line. Front-control and top-control machines, and panel-ready and stainless fronts, share the ranking above but not every part, and this page does not cover a specific model or control revision. Family- and model-specific pages, where they exist, are linked below.
Links: - [Common COSMO Dishwasher Problems](/blog/cosmo-dishwasher-problems) - [Cosmo Dishwasher: Abnormal Noise or Vibration](/blog/cosmo-dishwasher-abnormal-noise-2) - [Cosmo Dishwasher: Does Not Clean or Dry](/blog/cosmo-dishwasher-cleaning-performance-2) - [Cosmo Dishwasher: Controls, Display or Error Code](/blog/cosmo-dishwasher-control-fault-2)
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
