Why is my dishwasher leaving things dirty?

Filter, spray arms and loading — three causes in order of likelihood, and all three are free to fix.

Difficulty: beginner
Time: 40 min
Read: 3 min
Safety: caution

Short answer

Clean the filter and check the spray arms spin freely and their holes are clear. Between them they account for most dishwashers that have stopped cleaning, and neither costs anything.

A dishwasher that has gradually got worse is nearly always blocked rather than broken. The filter and spray arms are designed to be cleaned by the owner, and almost nobody does.

Safety

Turn the dishwasher off at the socket before reaching inside the sump or removing the spray arms. Dishwasher detergent is strongly alkaline and burns skin and eyes — wear gloves when handling residue and never mix it with other cleaners.

Step by step

Clean the filter.Bottom of the machine, twists out. It collects food debris and grease, and a blocked one recirculates dirty water over everything. Rinse under a hot tap with a brush. Do this monthly.
Check the spray arms spin freely.Turn each one by hand. If it catches on a tall item or a rack, that item was blocking the wash. This is a very common cause of one area coming out dirty.
Clear the spray arm holes.Take the arms off — they usually unclip or unscrew — and poke each hole through with a cocktail stick. Limescale and debris block them progressively.
Look at how you are loading it.Nothing overlapping, bowls and cups angled down, nothing blocking the spray arms, and large items not shielding smaller ones. Overloading is the most common single cause.
Check the detergent and where it goes.The dispenser must open freely — an item leaning against it stops it. Old or damp tablets do not dissolve properly. And check the rinse aid and salt if your machine takes them.
Run a hot maintenance cycle.Empty, on the hottest programme, with a dishwasher cleaner. Grease and limescale build up in the pipework and spray arms and reduce performance gradually.
Check the water is getting hot.A cycle that finishes with the dishes cool suggests a heating problem. This is where it becomes an engineer's job rather than a cleaning one.
Use a longer, hotter programme.Eco and quick cycles are shorter and cooler and struggle with anything baked on. For a dirty load, use the proper programme.

Tips

Scrape plates but do not rinse them thoroughly. Modern detergents need something to work on, and a completely pre-rinsed load can come out worse.
In a hard water area, check the salt level even if the machine says it does not need salt. Limescale is the main long-term killer of dishwasher performance.
If only the top rack is dirty, the upper spray arm is blocked or something is obstructing it. That localisation is a useful clue.

If it doesn't work

Gritty residue on everything

Cause: Blocked filter recirculating debris — Fix: Clean the filter thoroughly and do it monthly.

One area of the load is dirty

Cause: Spray arm blocked or obstructed — Fix: Check that arm spins freely and clear its holes.

White film on glasses

Cause: Hard water and no rinse aid or salt — Fix: Top up salt and rinse aid, and run a descaling cycle.

Detergent tablet still in the dispenser

Cause: Dispenser blocked by an item, or door not opening — Fix: Reload so nothing leans against the dispenser.

Questions people ask

How often should I clean a dishwasher filter?

Monthly with normal use. A blocked filter recirculates dirty water over the whole load and is the most common reason a dishwasher stops cleaning.

Should I rinse plates before loading?

Scrape them, but do not rinse thoroughly. Modern detergents are designed to work on food residue and a completely clean load can come out worse.