Is it allowed to use a dishwasher that has a timer on Shabbat? We are using the dishes both on Fri night and Sat lunch and need to wash them after dinner Fri night. We can put in the soap and set the dishwasher before Shabbat to go off in the middle of the night when everyone is sleeping.
According to the opinion of the great Poskim a dishwasher should not be used on Shabbat even if it is on a timer. It is prohibited for a number of reasons. Dishwashers are usually made in a way that they do not work when the door is open, so closing the door causes Mavir (lighting) and Bishul (cooking) on Shabbat. If it is a machine that works even when the door is open, but the water does not enter until the door is closed it is also forbidden. In some cases it may cause Bishul and Nolad in the remnants of the food. Some prohibit it because of Ziluta D'Shabbat (disrespect of Shabbat). [שש"כ יב, לה והערה צה, מ"ב סימן רנג ס"ק ק, אג"מ או"ח ד סימן ס, ספר מאור השבת ח"ד סימן יד סעיף קלח ובהערות, ושם בפניני המאור במכתבי הגרש"ז אויערבאך זצ"ל מכתב מה אות ג ובהערות].