Of all the ways an owner can describe a dishwasher problem, “the bottom rack is fine and the top rack is gritty” is the most useful. It rules out most of the machine before anyone opens the door.
Water reaches both racks from the same pump. If one rack is being washed properly, the pump, the heater, the inlet valve and the wash cycle are all doing their jobs. The fault is downstream, in the path that serves the rack that is not getting clean.
How an upper wash arm dies
The upper arm is fed through a column at the back of the tub and spins on its own bearing. Its holes are angled to throw water outward and upward across the rack. The holes toward the outer end of the arm cover the perimeter of the rack — the glasses at the corners, the bowls at the ends — and they are the holes that suffer first.
Two things work against them. Every hot cycle leaves a little deposit wherever water is leaving a narrow opening at speed, and food residue that gets past the filter arrives at the same openings. And the outer holes carry the lowest pressure margin, because pressure falls along the arm from the centre outward.
So the outer holes close first, one at a time, over years. The arm still spins. It still sounds exactly as it always did. It still washes the middle of the rack. The perimeter quietly stops getting cleaned, and because it happens slowly nobody notices a day on which it changed.
The lower arm is fed directly, runs at higher pressure and has larger holes. It carries on working long after the upper one has degraded. Hence the pattern.
Do the check yourself
Take the upper arm off — on most machines it releases with a nut or a quarter turn — and hold it up to a light. Count how many holes you can see through. Compare the outer holes with the inner ones. If the outer third of the arm is blind, you have found your fault, and you found it in four minutes.
While the arm is out, take the filter assembly out too. Thermador’s filter is fine-meshed by design, which is what makes the machine quiet and thorough, and it is also an efficient deposit and grease trap. A filter that has not been out in two years starves the pump, and the upper arm — furthest from it, lowest in pressure margin — is the first thing to feel it.
A soak in the cleaning solution specified for the appliance recovers a partially blocked arm. An arm whose holes have been closed long enough to have deposit through the full thickness of the wall usually does not come back, and replacement is a cheap part.
The film question, while you are in there
Owners who report grit usually report film as well, and the two need separating.
Mineral film wipes off with a little acid, or clears after a citric acid cycle. It is deposit sitting on the surface, and it is a water and rinse-aid matter rather than a fault.
Etching does not come off, ever. It is damage to the glass surface itself, and nobody can recover an etched glass — not us, not a polish, not a longer cycle.
This matters because the instinctive response to film is more detergent, and over-dosing at temperature is how film turns into etching. Keep the rinse-aid reservoir filled and dose for the load actually in the machine rather than for the worst load you ever run.
What to measure while you are at it
Two numbers make the visit shorter. The first is the cycle time: run the machine empty from a cold start on the same programme twice and write down how long it takes. A cycle that has stretched from two hours to four is almost always a heating fault, because the machine will not advance until the water reaches temperature — it is not slow, it is waiting.
The second is the door. Close a strip of paper in it at four points along its length and pull each one out. Even resistance at all four means the seal is compressing properly. Slack at one corner is the hinge or the spring rather than the gasket, and on a flush-fit installation it is often the custom panel: a panel changed during a kitchen refresh without the spring being retensioned will make a perfectly good door behave like a failing one.
When to call
If the arm is clear, the filter is clean and the top rack is still gritty, the fault is in the feed to the arm or in the arm’s bearing, and that is our job. So is standing water in the sump, which runs filter, then drain hose loop, then the air gap or disposer connection, then the drain pump — and the disposer knockout plug is worth naming, because a machine plumbed into a disposer whose knockout was never punched out will not drain at all.
Photograph the rating plate before you call. Wash arms, pumps and door components change across production runs, and the serial number is what lets us check the part before the van is loaded.
Flat $129 diagnostic, credited toward the repair. Written estimate first.