Dishwasher Not Cleaning: Spray Arms, Filter, or a Pump Fault?

A dishwasher that stops cleaning properly almost never announces the reason. There is no error code for “the jets are half the diameter they were in 2019.” The cycle runs, the machine sounds normal, and the dishes come out slightly worse every year until someone finally decides the appliance is worn out.

Often it is not. Here is how to work out which of the two common causes you actually have — mineral deposit or a mechanical fault — before you spend money on either.

The physics of a dishwasher, briefly

A dishwasher cleans with mechanical energy, heat, chemistry and time. The mechanical part is a pump pushing water through a fixed number of small holes in a rotating arm. The reaction of water leaving those holes is what spins the arm, and the velocity of the jets is what removes soil.

Everything in that sentence is sensitive to hole diameter. Cut the effective area of the jets and you lose velocity where it matters, you change the balance that makes the arm rotate at the right speed, and you change how pressure is shared between the lower arm, the upper arm and the ceiling spray. Nothing breaks. Performance just degrades.

Where the restriction actually forms

Two places collect deposit, and only one of them is visible.

The heating element, because precipitation happens fastest at the hottest surface. Deposit is a thermal insulator, so a coated element has to run hotter to move the same heat into the water, which makes it deposit more on itself — a slow feedback loop that ends in a local hot-spot failure. On machines with an exposed element in the tub floor you can see it as a chalky crust or a discoloured band. On the many premium machines that use a flow-through heater built into the pump housing you cannot see it at all. You only see the symptom, which is a cycle that runs long and finishes with cool, wet dishes because the machine never reached its target temperature in the time allowed.

The spray arms, because a little water sits in them between cycles and evaporates out. The deposit forms on the inside of each jet, where nobody looks, and it narrows the hole concentrically. A jet at half its original diameter has lost three quarters of its area.

There is a third restriction that is not deposit at all, and it is the one we find most often: the filter stack. The fine filter and the coarse screen sit in the sump and twist out by hand on nearly every premium machine. What collects underneath is a mixture of food soil, grease and fine grit, and a partly blinded filter starves the circulation pump in a way that produces exactly the same complaint as weak arms. It is a five-minute job with no tools and it belongs at the front of any inspection, not the end.

One more thing worth ruling out before any of the above: inlet temperature and fill volume. A machine that is filled with cold water because nobody ran the tap first, or that fills short because the inlet valve is metering badly, will underperform with a perfectly clean tub, perfectly clear arms and a perfectly healthy pump. The controls do not report either condition as a fault; they simply run the cycle they were given.

Telling a restriction from a pump fault

This is the part worth doing carefully, because the two produce a similar complaint and completely different repairs.

Observation	Points at a restriction	Points at a pump, diverter or supply fault
Water noise during wash	Normal, strong rush	Weak, pulsing, or a hum with little water sound
Cycle length	Normal, or long only if heating is affected	Often extended as the control retries
Wetting pattern inside	Even, but soft	Dry quadrant, or one rack untouched
Arms spin by hand	Freely	Freely — this does not distinguish them
Jets held to the light	Visibly narrowed or blocked	Clear
Noise from the sump	None unusual	Grinding, rattling, or a loud hum
Result after clearing jets and descaling	Noticeably better	Unchanged

The last row is the decisive one and it costs an hour rather than a part. Clear the jets, wash the filter, run an empty hot cycle with a dishwasher descaler, then run one real load. If nothing improved, stop replacing consumables and have the pump, the diverter and the fill checked.

One more distinction worth knowing: a dirty top rack with a clean bottom rack is rarely general low pressure. It is a feed problem — a blocked upper feed tube at the back of the tub, an upper arm that is fouling on a tall item, or a diverter valve that alternates spray between the upper and lower circuits and has stopped alternating.

Doing the inspection

Pull the lower arm, which on most machines lifts off or releases with a single retainer. Hold it up so light passes through each jet and compare them along the arm. This comparison is the trick — in isolation a half-blocked hole still looks like a hole.

Do the same with the upper arm and any ceiling spray. Clear blocked holes with a wire or a toothpick, working from the outside in, then flush the arm under a tap and watch water come out of every hole with a similar throw. Soak both arms in warm white vinegar or a dishwasher descaler for half an hour to lift what a wire cannot reach.

Do not use a drill bit or a needle. Enlarging one jet permanently unbalances the arm’s rotation and the pressure distribution across the rest of it, and there is no way to undo that.

Then pull the filter stack. The fine filter and the coarse screen twist out on nearly every premium machine, and the fine grey sludge underneath is a mix of food soil and precipitated mineral. A partly blinded filter starves the pump and produces symptoms that look exactly like weak arms, which is why it belongs in the same inspection.

The feature most owners never switch on

Many European dishwashers — Miele and Asko among them — include a built-in ion-exchange softener with a salt reservoir and a water setting in the control menu. Cove builds a model with an integrated softener for the same reason. It treats the fill water before it ever reaches the element or the arms, which is a far better place to solve the problem than downstream of it.

Two things go wrong with it. Owners do not know it exists and never fill it, or they fill it and leave the setting on its shipping default, which is chosen for water much softer than most supplies. Set the value to what your supply actually needs, and use dishwasher salt specifically — table salt and water-softener pellets can foul the resin bed and the reservoir.

If your machine has no softener, treating the supply upstream does the same job for the whole house, and the dishwasher is rarely the appliance that justifies it on its own.

What routine maintenance actually looks like

Once a month, pull and wash the filter stack. Twice a year, pull both spray arms and check the jets against a light. Twice a year, run an empty hot cycle with a dishwasher cleaning product. Keep the rinse aid reservoir filled — it will not stop deposits forming inside the machine, but it does stop mineral spotting on glassware, which is the symptom most people notice first.

And treat a white film on glasses as information rather than an annoyance. It is the same deposit that is currently insulating your heating element, arriving in the one form you can actually see.

We are an independent appliance repair company working across Duval, St. Johns and Clay counties, and a surprising share of “the dishwasher is dying” calls end with a clean filter stack, two clear spray arms and no parts at all.