The Dishwasher FilterYou’ve Never Cleaned

Somebody tells you the dishwasher is going. The glasses come out with grit on them, there is sand in the bottom of a bowl, and the whole load smells faintly of last night. Before you price a new machine, take the filter out and look at it. There is a good chance nobody has, and there is a good chance that is the entire problem.

Your machine probably has one. Your mother’s didn’t.

Older dishwashers dealt with food scraps by grinding them up. There was a little blade assembly under the bottom spray arm and it chewed everything small enough to go down the drain. It worked, and it was loud — that grinding was most of the noise an old dishwasher made.

Quieter machines meant losing the grinder. What replaced it is a filter: a fine mesh cylinder that catches the scraps instead of pulverising them. It is a better system in every way except one. A grinder looked after itself. A filter is maintenance, and it only works if somebody empties it.

If your dishwasher is quiet, it almost certainly has a filter. If you have never removed it, it has been collecting since the machine was installed.

Where it is, and what a bad one looks like

Pull the bottom rack out. In the floor of the tub, usually under the centre of the lower spray arm, there is a cylinder with a coarse mesh around it. Most twist a quarter turn counter-clockwise and lift straight out. Some have a flat plate around the cylinder that lifts out afterwards.

A filter doing its job looks dirty. A filter causing your problem looks packed — the mesh greyed over so you cannot see light through it, often with a layer of soft paste underneath and sometimes a shard of glass or a fruit sticker in the well below.

Rinsed under a hot tap with an old toothbrush, it comes back. This is the one piece of a dishwasher the manufacturer expects you to take out and clean yourself — the manual will say so.

Why a blocked filter makes dishes dirtier

A dishwasher does not fill with clean water and rinse. It fills a small well at the bottom, and then pumps that same water around the spray arms for the whole wash. The filter is what keeps that circulating water clean enough to wash with.

When the mesh blocks, the water going back to the pump is carrying the last load with it. The machine is then spraying dirty water at your dishes at pressure. That is where grit on a “clean” glass comes from, and it is why the problem gets worse in steps rather than all at once.

When it is not the filter

Clean the filter first, run a load, and see. If the grit is gone, you are done and it cost you ten minutes.

If it comes back quickly, or never improved, the next things worth looking at are the spray arms and the water temperature. Spray arm holes block with the same debris the filter is catching — take an arm off and hold it up to the light. And a dishwasher washing with lukewarm water will leave film on everything no matter how clean the filter is.

A machine that leaves standing water in the sump after a finished cycle is a different fault again, and worth having someone look at rather than guessing at.

The filter is the cheapest thing in the machine to check and the one most likely to be the answer. Look at it before you shop.