I was getting U1 error fairly frequently. There was foam in the bottom of the dishwasher. I figured the pump was not emptying machine properly. I cleaned all the muck out of the bottom of the machine... problem sloved.
Remove both shelves; remove spray arms (nut in centre); remove upper arm feeder tube (horseshoe clip on bottom of pipe assembly). remove seive tray and clean everything (with an old toothbrush). re-assemble...bingo!

Andrew Osborne, February 2009