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1) It is prohibited to place meat and dairy dishes together at one time in a dishwasher.

If one does so the utensils risk becoming non kosher.

Moreover, merely doing say may be a transgression of the biblical prohibition to cook milk and meat together, as if food particles of both food classes are in the dishwasher they can be heated up together. (Merely cooking meat and dairy even when not intended for consumption is prohibited) (See Igros Moshe Yoreh Deah Vol. 2 Siman 28 and 29 at length.)

2) It is best never to use one dishwasher for both meat and milk utensils, even separately and consecutively.

However, if one finds themselves in a situation where they need to use the same dishwasher for both meat and dairy utensils consecutively, there are certain conditions under which this can be done, as follows

* The dishwasher must be inspected for any food particles, and deemed thoroughly clean.

* The dishwasher may not have any parts that are constructed of ceramic or porcelain. (See Tur Yoreh Deah Siman 121 quoting the ruling of the Ba'al Ha'Itur)

* Separate racks on which the dishes will lay must be purchased and used for each food class being washed.

*Between the two washes, the dishwasher must be run with no utensils inside on the highest heat setting. (See Igros Moshe ibid. and Yoreh Deah Vol. 3 Siman 10 and 11. See also Aruch HaShulchan Yoreh Deah 121:18, Biur Halacha Siman 452 end of Dibur Hamaschil Tzarich and Chayei Adam Klal 125:32)

It is important to note that before relying on the above, a Rav must be consulted to discuss if any specific details or situations allow the above leniencies to apply or not.

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