The filter, where residual food often gets stuck, must be cleaned thoroughly. Then the dishwasher should be run at its hottest setting, so that any absorbed ĥametz is released, ke-bole’o kakh polto. Regarding the racks, le-khatĥila they should undergo hagala or irui with boiling water or be replaced. If it is difficult to kosher them through hagala or to replace them, one may perform hagala by running them through the dishwasher’s longest and hottest setting.
In any event, one must wait twenty-four hours after the last load of ĥametz utensils before using the machine with Pesaĥ utensils.
Some take a stringent approach to dishwashers and consider them to have the status of a kli rishon on a flame. This means that to kosher a dishwasher one must put a white-hot piece of metal in it in order to boil the water. However, those who follow the lenient approach have authorities on whom to rely.
. The highest temperature of the water in the dishwasher is c. 80ºC. A dishwasher absorbs at the level of irui from a kli rishon (see above 10:8), since the water is heated by an element (the kli rishon) and then sprayed at the dishes. Thus, ke-bole’o kakh polto applies. Nevertheless, the custom is to kosher all utensils in a kli rishon on a flame; thus, Igrot Moshe OĤ 3:58 states that one should place a scorching hot stone in the dishwasher. When this is difficult, one may perform hagala using the mechanism of absorption, because most poskim maintain that one must perform hagala using boiling water only when koshering a kli rishon used on the flame; when koshering a utensil that absorbed via irui from a kli rishon, the hagala water only needs to reach yad soledet (see above ch. 10, n. 10). This is certainly the case vis-à-vis the dishwasher, which is koshered at least at the same temperature at which it absorbed. Technically, the racks may be koshered in this manner, but since they actually came into contact with food, some authorities ruled that they must be koshered via hagala in boiling water, or at least via pouring boiling water on them (Igrot Moshe). Additionally, some authorities consider the dishwasher to be akin to a kli rishon on the flame; according to these authorities, one must kosher the dishwasher at the intensity of a kli rishon on the flame. According to these authorities, one may only be lenient vis-à-vis the walls of the dishwasher, which do not usually come into contact with food, but not vis-à-vis the racks (R. Pfeiffer’s Kitzur SA, Basar Be-ĥalav vol. 2, explanations 6 and 7). Notwithstanding this stringent view, the mainstream opinion is that the absorption was at the level of irui; therefore, when alternatives are difficult, one may kosher the racks inside the dishwasher. This is the opinion in Hagalat Kelim 13:225-228. Additionally, several authorities maintain that the principle of ke-bole’o kakh polto applies to the temperature of the absorption in a kli rishon, so certainly the dishwasher’s highest setting is sufficient to kosher the trays (see SAH 451:25 and Sidur Pesaĥ Ke-hilkhato 1:4).
Igrot Moshe maintains that if the body of the dishwasher is made of porcelain, it cannot be koshered (this is very uncommon; see also below n. 11 that there are those who are lenient, especially since the food does not touch the walls and it is likely that the dishwashing detergent ruined any taste even before it was absorbed. Thus, there is room to be lenient, as with a sink). See also Sidur Pesaĥ Ke-hilkhato 8:32, which rules stringently and requires one to use a white-hot stone to kosher a dishwasher. Piskei Teshuvot 451:25 states that there is concern about koshering plastic, as some poskim maintain that it is impossible to kosher plastic (ibid. 53). However, the mainstream opinion and the view of most poskim is that plastic can be koshered via hagala, plus this is a situation of uncertainty regarding a rabbinic prohibition, as twenty-four hours have elapsed since the dishwasher’s last use. Hagalat Kelim 13:91 concludes that poskim maintain that plastic utensils can be koshered. R. Dov Lior is lenient regarding all types, as I have written.