Spray arms, seals and the door edge
Next, spin the spray arms with your hand. They should move freely. Hold each one up and check the little holes: if any are blocked with limescale or seeds, poke them clear with a cocktail stick or a straightened paperclip. Blocked jets are the reason a machine washes well on one side and leaves the other side dirty.
Then do the part nearly everyone skips: the rubber seal around the door and the lip at the very bottom of the door frame, below where the water reaches. Wipe along the folds of the seal with a damp cloth and a little washing-up liquid, and go around the door edge with the same cloth. Black marks and a sour smell almost always live here rather than inside the machine. Finally, wipe the detergent dispenser flap, which gets sticky and stops opening fully.
The hot empty cycle that finishes the job
With the filter back in and the arms clear, run the machine empty on the hottest cycle it has. A cup of white vinegar in a bowl on the top rack helps cut grease and light scale; if you prefer, use a proprietary dishwasher cleaner and follow the instructions on the bottle. Do not combine the two, and do not add vinegar to a cycle with dishes in it.
Once a month is plenty for the hot empty run. The filter, though, deserves a rinse every week or two if you cook a lot, and thirty seconds with a cloth around the seal each time you unload keeps the smell from ever starting.
