A warming tray (“plata”) is forbidden on Pesaḥ because of baked goods that are placed on it and because of cooked foods that sometimes spill on it. Since it absorbed through fire, it must be kashered through libun. However, libun will damage it, and as we learned, when necessary, kelim that absorbed through fire can be kashered by heating them up to the temperature at which the absorption took place (above, 10:5 and 10:3). Therefore, one may kasher a plata by cleaning and heating it up for an hour. Those who are stringent also cover it with aluminum foil to create a barrier between the plata from the Pesaḥ pots.
A grill: The body of the grill and the racks are kashered the same way they are used, which is at the intensity of heavy libun. If it is a gas grill, it is kashered at the highest temperature setting; if it is a charcoal grill, it is kashered by putting in the largest amount of charcoal that one regularly uses.