The short answer
Water standing above the filter at the end of a cycle on a Bosch SMU-series dishwasher is almost never the drain pump, whatever the noise suggests. On this platform the path is short and the things that close it are ranked by the full-width cabinet and by how the machine was plumbed in: the softener reservoir and an undercounter fit both change which end of that path you should look at first.
What you observe
- A stale smell from the tub between cycles — Standing water that has been there longer than one cycle.
- The sink beside it drains slowly too — That moves the whole diagnosis out of the machine and under the worktop.
- Water reappears in the tub hours after a cycle — Not a drain failure at all but a back-feed, and it is diagnosed from the other end.
- NEAR MISS - the machine drains continuously and refuses to fill — That is leak protection reporting water beneath the tub, not a drain fault. It belongs on the leak page and it does not get better by cleaning a filter.
Likeliest causes here
- The filter and the sump beneath it — Everything the wash lifts off the dishes arrives here, and the drain path starts on the far side of it. First on this family, and for a reason that belongs to the platform: the sump on this cabinet holds less than a full-size one, so it closes on less.
- The drain hose kinked or crushed behind the machine — A hose squashed when the machine was pushed back into its opening will never clear itself. Second here, and close enough behind the first to be checked in the same pass: getting at the back of this installation is the hardest part of the job, which is why it is checked before anything is dismantled.
- Something solid in the drain pump impeller — Glass, a fruit stone, a bottle label or a bone fragment reaches the impeller and jams it, and the motor hums against it. Ranked third on these models: it announces itself with a hum, so it is rarely a mystery.
- The air gap blocked, or a disposer knockout left in place — The path above the worktop closes as readily as the path below it, and a new disposer that has never had its plug punched out blocks it completely. Fourth here. A fixed installation that has drained before rarely develops it.
- A non-return flap held open by debris — Once the flap cannot seat, what the pump sent out comes back in. Fifth on this family - uncommon, and it presents as water returning rather than water never leaving.
- The drain hose not looped high enough — Without a high loop the tub and the sink drain find their own level, and the machine fills itself back up after it has finished. Last, and the ranking is the point: it produces refilling rather than a failure to drain.
- Where this ranking departs from the brand-generic page — The cause that moves is the filter and the sump beneath it. It leads on this family because the machine slides under a worktop that is not its own, with the plumbing crowded in behind it. That is a property of the platform rather than of the brand, which is why the page above this one cannot rank it the same way.
Check in this order
- Bail the tub out, then twist the filter assembly free — Clear the sump by hand and look for glass. What is under the filter is the diagnosis more often than anything downstream.
- Look at the hose where it leaves the cabinet — A crush mark or a tight bend is visible without pulling the machine right out.
- Reach into the sump opening and turn the impeller — It should move freely. Anything gritty or seized is what the hum has been fighting.
Before you open anything
Safety
Unplug the appliance or switch off its breaker before removing any panel or reaching under the tub - a capacitor can hold a charge after the power is off. Bail the standing water out before you start rather than after, assume broken glass in the sump, and wear a glove for the impeller.
When to call
Everything inside the tub is owner ground on these models, and so is everything under the sink. The line is the base plate: a pump that is silent when it should run, a control that never calls for a drain, or a leak-protection state that will not clear are all diagnosed with a meter and the machine tipped.
Nearest pages
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
