My Bosch dishwasher was having problems filling. No fill, and the drain motor would continuously run. A web search turned up this post to alt.home.repair, though I can't find the original thread in this NG.
Anyway, the remedy described here also worked for me. Many thanks to 'mcquarrie'!
Original post:
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Date: 15 Jun 2004 11:04:51
>From: mcquarrie
>Subject: Bosch dishwasher SHU 3300 cycle not advancing past initial drain repair story
>
>machine got stuck trying to drain water for 6 hours before starting a
>wash cycle, hope it didn't damage the drain motor. However...
>
>i could see water was being pumped out of the dishwasher by looking
>into the waste disposal, flowing fine so no blockage in drain... >
>incoming water pressure confirmed OK, by disconnecting incoming water
>pipe and flowing into bucket...
>
>removed baskets and spray arms & started removing plastic parts from >inside tub.
>removed plastic tube which feeds water to both spray arms (2 torx
>screws). then removed small plastic cover on LHS of drain sump (1 torx
>screw). this exposed the (i think ) drain impeller, which was turning,
>not quite freely, but in a way that tells me there is still magnetic
>force between the rotor and stator of the drain motor.
>
>also exposed is a small black rubber object, which was gunged up with
>powder; removed that from its position inside a narrow slot with a
>screwdriver, and it's actually a little flapper valve with a round
>seal about the size of a nickel. cleaned off the powder, rodded the
>drain port with a bottle brush, and flushed the drain a few times with
>hot water. slipped the seal back in its slot, put everything back
>together, and cycle advances normally. BINGO!
>
>Not sure about this, but I think the flapper stops water coming back
>into the drain sump (the drain pump can't get ALL the water out, some
>will inevitably remain in the drain hose between the flapper and the
>air-gap) otherwise the float switch will never drop down telling the
>machine "ok im drained, go to next step". So if the flapper is gunked
>up, not sealing, water is coming back in from the drain hose as fast
>the pump is pushing it out, so the float switch stays up, so the drain
>keeps on pumping.