To kosher an oven, clean it thoroughly and run it at its highest setting for half an hour.
It is difficult to kosher baking trays. Because they absorb through fire, they require heavy libun, but since heavy libun will cause them serious damage, they may not be koshered (see above 9:7). One must therefore buy special baking trays for Pesaĥ, while the ĥametz trays must be cleaned and put away like all other ĥametz utensils. If one does not have Pesaĥ trays, he may use disposable trays. However, he must also kosher the racks along with the oven and cover them with aluminum foil, and only then he may place the disposable trays on the racks.
Ovens that self-clean at a temperature of 500ºC need not be cleaned before koshering because such intense heat is considered heavy libun and is sufficient to kosher the oven for Pesaĥ.
. Some are stringent and insist that ovens cannot be koshered for Pesaĥ, due to a concern (present in older ovens) that crumbs would fall into the door of the oven before Pesaĥ and mix into the Pesaĥ food. The solution to this problem is to clean the door of the oven with a lot of soap, thereby befouling the taste of any crumbs stuck there and rendering them unfit for a dog’s consumption. Others are stringent out of concern that perhaps food touched the walls of the oven, which would mean the oven would need to be koshered with heavy libun – impossible without damaging the oven. But most poskim rule that ovens can be koshered since the oven itself rarely comes into contact with food and only absorbs steam from the food. Light libun at the oven’s highest temperature would certainly kosher it. Even if some ĥametz spilled and its taste was absorbed in the walls of the oven, we have seen that SA (451:6) permits koshering utensils (and ovens) based on the main usage. Even MB (451:48) is lenient in such a case, provided that one performs light libun. We can also add the opinion that the absorption of ĥametz before Pesaĥ is considered heteira bala, for which light libun is effective even le-khatĥila (see above 10:6). Finally, we may factor in the opinion that ke-bole’o kakh polto applies to the temperature of the libun (see above 10:5), so heating the oven at its highest setting is effective.
With regard to baking trays, however, we are stringent and require heavy libun. However, if one conducts light libun on a tray, he may place a disposable tray inside of the multi-use tray, and certainly atop the racks. It is best to cover the racks with aluminum foil, so that if something spills onto them it will not connect the Pesaĥ tray to the insufficiently koshered racks. One need not wait twenty-four hours before koshering them, since light libun releases and incinerates the taste absorbed in the utensil. See Sidur Pesaĥ Ke-hilkhato 8:3, which treats this issue at length and explains the entire process of koshering an oven.
For ovens that self-clean at 500ºC and incinerate all dirt, since they heat to a temperature that certainly would cause obsolete alloys to spark or redden (see above 10:5), this self-cleaning qualifies as heavy libun and koshers the oven. Baking trays may also be koshered at this heat, but manufacturers’ instructions caution that leaving trays in such heat can ruin them. Consequently, they cannot be koshered by this method (see above 10:7).