Dishwasher not draining: causes and what to check

Before you start

Switch the dishwasher off at the wall and let hot water cool. Take out the lower basket. If there is a lot of standing water, scoop it out with a cup into a bowl and soak up the rest with a sponge or towel, so you can see the filter and the sump beneath it.

Do not keep restarting the cycle. If the problem is the sink waste, each pump-out sends more greasy water back into the sink or the cupboard under it.

Where the fault usually is

Most dishwashers drain through a hose that runs under the units into the sink waste. That shared route is why kitchen drain problems so often show up as a dishwasher fault.

Clean the filter and sump

Most dishwashers have a cylindrical filter in the base that twists out anticlockwise, often with a flat mesh plate around it. Lift both out and rinse them under a warm tap with a soft brush. Food, fruit stickers, bits of glass and fat are the usual culprits. Wear gloves, because broken glass often ends up here.

Look into the sump under the filter. Some models have a small cover over the drain pump that can be removed by hand according to the manual, so you can check the impeller turns freely. Anything jammed there, such as an olive stone or a shard of glass, will stop the pump. Put the filter back exactly as it came out, locked into place. A dishwasher run with the filter out of position can pass debris into the pump.

If the filter was badly clogged, clean it every week or two from now on. Scraping plates before loading helps more than rinsing them.

Check the drain hose

The drain hose runs from the back of the machine, under the units and up to the sink waste. Pull the dishwasher out gently, watching the hoses and the cable, and look for a kink or a flattened section. Integrated machines are often pushed back against a hose that has been folded behind the plinth.

The hose should rise to a high point, often clipped under the worktop, before it drops to the sink waste. That loop stops dirty sink water running back into the dishwasher. If it is lying on the floor of the cupboard, the machine can drain poorly and smell. Check the manufacturer's installation instructions for the height it needs.

Check the sink trap and spigot

Under the sink, the dishwasher hose joins a spigot on the trap or waste. Kitchen sink traps collect fat, coffee grounds and food, and a partial blockage that the sink copes with can be enough to stop a dishwasher, because the machine pumps a lot of water in a short burst.

Put a bowl underneath, undo the trap and clean it out. Check the rubber connector at the spigot too. On a newly fitted waste, look inside the spigot for a blanking disc: some are supplied sealed and have to be opened before an appliance is connected. Our guide on how to unblock a sink covers clearing the trap and the waste safely, and if the leak is under the sink rather than in the machine, see our article on a leak under the kitchen sink.

Avoid chemical drain cleaners in the dishwasher or its waste. They can damage seals and plastic pipework, and they are dangerous for anyone who then opens the trap.

If the sink, dishwasher and washing machine are all slow

When every appliance on the kitchen waste struggles, the blockage is further along: in the waste pipe through the wall or in the drain outside. Check the gully the kitchen waste runs into. If it is full or overflowing, the drain beyond it needs clearing. Our guide to an outside gully overflowing explains what to look for, and our article on a washing machine not draining covers the other appliance on the same waste.

Faults inside the dishwasher

If the filter, sump, hose and sink all check out, the fault is inside the machine: a failed or jammed drain pump, a blocked internal hose, or a non-return valve stuck shut. Most dishwashers show an error code for a drain fault. The codes vary by make, so look up yours in the manual or on the manufacturer's website. Internal faults belong to the manufacturer or an appliance engineer, particularly while the machine is under warranty.

What a plumber will do

A plumber will clear and check the trap and spigot, look at the hose route and loop height, test the waste pipe and trace any blockage out to the drain. Where the dishwasher was connected in a way that lets dirty water run back or siphon, they will reconnect it properly, and if the waste pipe has too little fall to carry the flow, they will explain what it takes to correct it. They then run a drain cycle and check every joint under the sink.

Call us about a kitchen that won't drain

Loyal Plumbers covers London and everywhere inside the M25, 24 hours a day, with 10 vans on the road. Call 020 4652 8585 and tell us whether the sink is slow too, whether water is coming up anywhere, and whether the kitchen or dishwasher was fitted recently.

Our call-out charge is £60, and the engineer explains what they have found and what the work involves before starting. You can read more about our kitchen plumbing work, or our emergency plumbing service if dirty water is overflowing.