A dishwasher cleans by force, not soaking: a circulation pump pressurizes water and drives it through the spray arms, which spin and blast every surface. When dishes come out dirty after a complete cycle, water either isn’t reaching them or isn’t hitting hard enough. That narrows the cause to the water-delivery chain — pump, spray arms, filter, chopper — plus two things that aren’t mechanical at all: water temperature and how the racks are loaded.

A useful first test: open the door mid-cycle and check whether the spray arms have moved from where they started. If they’re in the same position, water pressure isn’t turning them — a clog or a failing pump. If they’ve spun but dishes are still dirty, the arms are turning but the holes are blocked, the water’s too cool, or the load is wrong. That one observation splits the diagnosis in half.

The spray arms have rows of small jets, and hard-water scale and food debris clog them over time. A clogged jet kills the pressure that spins the arm, so cleaning suffers and the arm may stop rotating entirely. Pull each arm off — most twist or unclip — and check that it spins freely on its mount and that every hole is open. Clear the jets with a toothpick or thin wire, soak the arms in white vinegar to dissolve mineral scale, rinse, and reinstall. In hard-water areas this needs doing roughly monthly. This is the single most common cause of poor cleaning and it costs nothing.

Modern dishwashers grind food debris so it drains away instead of recirculating. On Whirlpool-family machines a food chopper — a small blade-and-screen assembly in the sump — minces particles before they reach the pump. When the chopper dulls, cracks, or clogs, debris isn’t broken down, doesn’t drain, and gets sprayed back onto the dishes: the classic “food redeposited on clean dishes” complaint. Accessing it means removing the lower spray arm and spray tower; the blade has sharp edges. Alongside it sits the filter, which on many machines must be removed and rinsed periodically — a neglected filter chokes water flow to the arms and is a frequent cause of gritty results. Cleaning the sump, chopper screen, and filter with a shop vacuum and a rinse restores flow.

If the arms are clear but won’t rotate, the circulation pump is the suspect. The pump’s motor drives an impeller that forces water up into the arms; if the motor is failing, the impeller is broken, or the housing is clogged, there’s no pressure to spin them. The tell, from the mid-cycle test above, is spray arms sitting exactly where they started. A pump that hums after the fill but does no work points at a burned-out motor or jammed impeller. On the Whirlpool family the circulation pump and motor assembly is part W11414076, around $99, and it’s the one genuinely expensive cleaning-related repair. (Don’t confuse it with the drain pump, WPW10348269, which empties the machine — a drain pump fault leaves standing water, not dirty dishes.)

Two smaller parts cause specific patterns. The water inlet valve (Whirlpool W10648041) lets water in; if it’s clogged or weak, the dishwasher under-fills and there isn’t enough water to clean. A clogged valve is replaced, not cleaned — cleaning makes failure more likely. And the upper discharge housing gasket, which sits above the circulation impeller, can come loose or crack, bleeding off pressure meant for the upper arm — the signature is a clean bottom rack and a dirty top rack.

This is where many “broken dishwasher” calls turn out to be nothing wrong with the machine. Dishwasher detergent needs heat to dissolve and activate: the incoming water should be at least 120°F, and the main wash typically runs 130–140°F. If your water heater is set low, or the kitchen tap hasn’t been run hot before starting, the detergent never fully works and dishes come out filmed and gritty. Run the kitchen hot tap until it’s hot before starting the cycle, and set the water heater to at least 120°F.

Detergent itself changed in a way that still drives complaints. In July 2010, sixteen states restricted phosphates in dishwasher detergent to 0.5 percent, down from as much as 8.7 percent, after the ingredient was tied to algae growth in freshwater — a shift documented by Consumer Reports. Phosphates were what cut through grease and rinsed away mineral deposits, and reformulated low-phosphate detergents clean noticeably worse in hard and cooler water. The practical fixes: use fresh detergent (it loses potency when stale), always use a rinse aid to counter spotting and filming, and in hard-water areas consider detergent packs formulated for it. A great deal of “my dishwasher stopped cleaning” traces to this and to loading, not to a failed part.

These include our $99 service call applied to the repair.