Dishwasher Not Cleaning Dishes Properly: Why

What's actually happening

A dishwasher cleans with three things working together: water hot enough to dissolve grease, spray arms turning fast enough to reach everything, and detergent that has not been washed away before it can work. Poor results almost always trace to one of those three, and the majority are maintenance or loading rather than a broken part — which is good news, because it means most of this is fixable in an afternoon without a technician.

Check these first — they're free

1. Spin the spray arms by hand. They should turn freely without catching. Anything that stops them — a fallen utensil, a tall dish, a scaled bearing — costs you the whole zone that arm covers.
2. Clear the filter and check for scale in the jets. Twist out the filter, rinse it under a tap, and hold each spray arm up to the light to see whether the holes are open.
3. Run the kitchen tap hot, then start the cycle. This puts hot water in on the first fill instead of the cold that was sitting in the pipe. If results improve noticeably, your supply temperature was the problem.
4. Check the rinse aid and detergent. Empty rinse-aid reservoirs cause cloudy, spotty glassware, and detergent that has sat in a damp box loses potency. Both are common and both are cheap to rule out.

What causes it, most likely first

Spray arm holes blocked (most common, you can fix this yourself)

The jets clog with food debris and hard-water scale, and a partly-blocked arm leaves whole zones of the rack untouched. Take the arms off — usually a clip or a single nut — and clear each hole with a toothpick, then rinse them out. If dishes are clean at the front and dirty at the back, or clean on the bottom rack and dirty on top, this is the first suspect.

Filter clogged, so dirty water is recirculated (most common, you can fix this yourself)

The filter in the floor of the tub traps debris. Packed, it forces the pump to recirculate water full of the food it just removed, which is why dishes come out gritty rather than simply not clean. Modern dishwashers expect this filter rinsed monthly; most have never had it done.

Incoming water not hot enough (common, you can fix this yourself)

Dishwashers need roughly 120°F to dissolve grease and activate detergent. A water heater set low, or a long pipe run that starts the cycle with cold water, cripples cleaning. Running the kitchen tap hot before starting the machine is a genuine fix, not a workaround — it means the first fill is hot instead of cold.

Loading and pre-rinsing habits (common, you can fix this yourself)

Overlapping dishes shadow each other, tall items block the upper arm, and — counter-intuitively — rinsing plates too thoroughly leaves modern enzyme detergents nothing to grip, which actually worsens results. Scrape, do not rinse, and leave space between items.

Wash pump or diverter failing (less common)

If the arms turn weakly or not at all with everything clean, the pump is not generating pressure, or the diverter that alternates water between upper and lower arms has failed. This is the genuine mechanical fault behind poor cleaning, and it is the one that needs a technician.

When to call a technician

Call when the arms are clear and spinning freely, the filter is clean, the water is hot, and dishes still come out dirty — that points at the wash pump, the diverter, or a heating element that is no longer raising the water temperature during the cycle. Those are internal parts, and diagnosing them properly means measuring what the machine is actually doing rather than guessing from results.

How long can it wait? Nothing here is damaging the machine, so this can wait — with one caveat: a dishwasher that never reaches temperature is not sanitising, which matters more in a household with small children or anyone immunocompromised. If cleaning quality dropped suddenly rather than gradually, that is more likely a part than a habit, and worth looking at sooner.

What it costs

If you fix it yourself: $0 to clean arms and filter; $5–$15 for rinse aid and fresh detergent.

Professional repair: A fixed price after diagnosis, agreed before any work starts; the diagnostic fee is applied to the repair when you approve it.

See local dishwasher repair cost data

Frequently asked questions

Should I rinse dishes before loading?

Scrape, do not rinse. Modern detergents use enzymes that need food residue to work on — a spotlessly pre-rinsed load can actually come out worse, and you have used hot water twice. Remove bones and large scraps and let the machine do the rest.

Glasses come out cloudy. Is that the same problem?

Usually not dirt but minerals. Cloudiness that wipes off is hard-water scale — rinse aid and a periodic descale fix it. Cloudiness that does not wipe off is etching, which is permanent and comes from very hot water plus too much detergent on delicate glass.

There's grit on everything after the cycle.

That is the filter. Grit means the machine is recirculating water carrying the food it removed. Clean the filter and the sump thoroughly and the difference is usually immediate.

It worked fine until recently.

A sudden change points at a part rather than a habit — a diverter, a pump, or a heater that has stopped bringing the water up to temperature. Gradual decline over months is more often scale, filters and detergent. Which pattern you have is the most useful thing you can tell the technician.