You probably never knew the dishwasher had a filter until the morning the bottom of the tub held an inch of gray water and his coffee cup was still in it. Almost every dishwasher has one, tucked at the bottom of the tub, usually under the lower spray arm. It catches bits of food, seeds, and the grit that comes off plates, so the pump can keep pushing clean water over the dishes instead of recirculating sludge. Mom probably pulled it without thinking about it. Now it belongs to someone, and the most useful thing you can do is decide who.
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Here is how to find it. Open the dishwasher, empty the bottom rack, and look under the lower spray arm. Many filters are a round cylinder you twist counterclockwise and lift out. Others are a flat screen that sits in a frame. Your owner's manual will show the picture, and it is worth keeping a copy taped inside the cabinet door where he can see it, because he will not go looking for it in a drawer.
What to check tonight
Pull the filter and rinse it under warm running water. Use a soft brush, an old toothbrush works, to get the fine mesh clean. Do not scrub it with steel wool, because that can tear the screen. Then look at the spot in the tub where the filter sat. If food is stuck in the corners, wipe it out with a damp cloth. Put the filter back the way it came out, with the tab lined up, and if the smell is strong, run an empty cycle with a cup of white vinegar on the bottom rack.
Then decide who does this every week, and be specific. 'Whoever runs the dishwasher' is not an answer, because in a house with one man in it, the answer is usually nobody. Pick a day that is easy to remember, like trash day, and write it on the same calendar as the other weekly chores. If he is doing it, tell him it takes two minutes. If you are doing it on visits, say so out loud so he knows he does not have to think about it.
How you know it is full
The signs are plain once you know them. Water sits in the bottom of the tub after a cycle. Dishes come out with a film on the glasses or a gritty feel on the plates. There is a sour smell when he opens the door in the morning. The cycle runs longer, or the machine gurgles in a way it did not before. Any of these means the filter needs a rinse, and if a rinse does not fix it within a cycle or two, the drain may be clogged farther along.
When to call someone
If water still stands after you have cleaned the filter and checked that the drain hose is not kinked, call an appliance repair person. Ask three things before they come out: whether the visit includes checking the drain hose and the pump housing, what the visit covers if the fix turns out to be only a cleaning, and whether they will write down what they found. A written note is worth having when the next repair question comes up.
Keep the filter on the list. A dishwasher is a small machine that runs every day, and the people who keep it working are the people who know where the filter is. For more of the first stretch after a loss, the Start Here page walks through the order most families find useful.