Why does my dishwasher clean some loads but not others?

What intermittent dirty dishes actually tell you

Here's the useful part: a dishwasher that fails randomly is giving you a clue. If it were a dead pump or a burned-out element, every load would come out dirty. It doesn't.

That means something is marginal. It works when conditions are easy and chokes when they're not. Your job is to find the variable.

The big three variables are water flow (blocked by a dirty filter or clogged spray jets), water temperature (too cool to dissolve detergent), and load geometry (dishes blocking the spray).

Almost every intermittent case Techs have seen traces back to one of those. The good news is all three are cheap or free to fix yourself.

The most likely culprits, in order

Start with the filter. It sits in the floor of the tub, usually a twist-out cylinder. Pull it and look. If you see a gray paste of food and grease, that's your answer more often than not.

A clogged filter starves the pump of clean water to recirculate. Light loads don't produce enough debris to matter. Heavy loads do, and that's when dishes come out gritty.

Next, the spray arms. Each little jet can plug with a seed, a scale flake, or a chunk of label. When half the holes are blocked, the water can't reach the top rack or the back corner.

Spin them by hand. They should turn with almost no resistance. If one drags or stops, water pressure alone won't move it during a cycle, and whole sections of the rack get missed.

Water temperature is the sneaky one. Detergent enzymes need around 120F to work. If your heater is set to 110F or the first fill draws cold pipe water, the wash runs weak and nobody suspects it.

Then there's loading. I've watched people wedge a sheet pan across the bottom rack that blocks the lower arm completely. The dishes above it never get washed. It looks fine until you unload.

How to work through it, step by step

Do these in order. Each one rules out a suspect, and you'll usually solve it before the list runs out.

First, clean the filter thoroughly. Rinse under hot water, scrub the mesh with a toothbrush, and check the coarse screen for glass and bones. Reseat it firmly; a loose filter lets debris back into the pump.

Second, remove and clear the spray arms. Most pop off with a clip or a quarter-turn plastic nut. Hold each up to the light and push a toothpick through every jet.

Third, address water temperature. Run the kitchen hot tap until it's genuinely hot, then start your cycle immediately. Check your water heater is set to about 120F.

Fourth, replace tired detergent with a fresh product stored somewhere dry. Add a rinse aid if you're not using one, especially with hard water.

Fifth, run a cleaning cycle. Put a cup of white vinegar in a bowl on the top rack and run the hottest cycle empty. This strips scale from the arms and element.

Sixth, reload carefully and run a real load. If it comes out clean, you're done. If it doesn't, you've earned a pro diagnostic and you can tell them exactly what you already ruled out.

What this costs to fix in 2026

If it's the filter, spray arms, detergent, or loading, your cost is basically zero beyond a $6 bottle of vinegar and a fresh box of detergent.

If you decide to replace a spray arm yourself, a genuine part runs $30 to $65 depending on the brand and whether it's the upper or lower arm. That's the whole DIY range for this symptom, roughly $20–$110 including a cleaning kit.

A pro diagnostic visit typically runs $90 to $140 and usually applies toward the repair if you go ahead.

A spray arm or filter housing replacement done by a tech lands around $150–$500 with the visit rolled in. A wash pump or diverter repair is the pricier end, roughly $150–$500, because it means pulling the machine and opening the sump.

For context, a solid new mid-range dishwasher runs $550 to $900 installed in 2026. So if you're staring at a $330 pump job on a 10-year-old unit, replacement starts to make sense.

Under 8 years old, almost any repair under $250 is worth doing. The machine has plenty of life left.

What you can handle and what needs a tech

Cleaning the filter, clearing spray arms, checking water temperature, swapping detergent, and running a vinegar cycle are all firmly DIY. No tools beyond a toothpick and a toothbrush, and no risk if the power's off.

Replacing a spray arm is also DIY-friendly on most machines. It clips or twists off. If yours is held by a hidden bearing assembly, that's the line where it gets fiddly.

Anything involving the wash pump, the diverter valve, the heating element, or the control board should go to a licensed appliance tech.

Those repairs mean disconnecting water and power, tipping the machine, and working with wiring near a wet environment. A mistake there can flood a floor or damage the board.

The honest rule: if the fix is inside the sump or behind a panel with wires, call someone. If it twists out from the tub, you've got it.

Can this wait, or should you deal with it now?

This is a moderate-urgency problem. Nothing's going to flood your kitchen tonight. But putting it off makes it worse and grosser.

A neglected filter turns into a swamp within a couple of weeks. The smell alone will push you to act.

Mineral scale keeps hardening on the element and jets the longer you wait, so the eventual cleaning gets harder and less effective.

If you hear the pump straining or buzzing, don't wait on that part. A pump working against a clog can overheat and fail, and that's when a free fix becomes a $300 one.

For the cosmetic stuff (spotty glasses, occasional grit), you've got a few weeks of runway. Just don't let it become the new normal.

The hard-water angle most people miss

If you're in a hard-water region (much of the Southwest, Texas, the Midwest, and parts of the Mountain West), scale is a slow saboteur.

Calcium and magnesium deposit on every wet surface. Over a year or two, they narrow the spray jets and coat the heating element like limescale in a kettle.

The result is exactly this symptom: cleaning that gets gradually spottier and more inconsistent before anything actually breaks.

A monthly vinegar or citric-acid cycle keeps scale in check. A rinse aid helps water sheet off cleanly instead of drying into spots.

If your water tests above about 10 grains per gallon, a whole-house softener pays for itself across the dishwasher, water heater, and every fixture in the house.

How we put this guide together

This reflects patterns from years of appliance service calls where the complaint was intermittent, not total, failure. Those cases cluster hard around a few root causes.

Cost ranges are 2026 US estimates based on common parts pricing and typical regional labor rates for appliance techs. Your local rates will vary, and rural areas often run higher.

We lead with the cheapest, safest checks first because that's genuinely how a good tech works a call. No point pulling a pump when the filter's full of gunk.

When we say call a pro, it's because the repair involves water, wiring, or disassembly where a mistake costs more than the visit. Everything else, we hand you the toothpick and get out of the way.