How to kosher an oven.
An oven must be made kosher (kashered) before it can be used for Pesach (Passover). The normally accepted procedure for koshering an oven for Pesach is as follows:
- Thoroughly clean the oven using a caustic oven cleaner (like Easy-Off)  This includes the ceiling, floor, sides, corners, door hinges and edges, area behind the burners, and all grooves. If the oven has a fan, it should be “sprayed with a caustic cleanser while it is circulating, and cleaned to the best of one’s ability”. 
- Wait 24 hours.
- Heat the oven to a high temperature for a certain length of time (libun). There are various opinions as to the level of heat and length of time required as discussed below.
Koshering an object by libun means heating it to a high temperature so that any absorbed forbidden substances are either drawn out or totally incinerated. Utensils used to heat food over a fire without liquid, such as our ovens, can be made kosher for Passover only by libun.
There are 2 levels of libun:
- libun gamur or chamur (heavy libun): the heated object glows red or emits sparks. The temperature required for this is about 900 degrees Fahrenheit or 485 Centigrade. Libun chamur is required when the food comes into direct contact with the hot utensil. It totally incinerates any of the food residues that may have been absorbed by the utensil.
- libun kal (light libun): a piece of straw burns when placed on the opposite side of the heated object. The temperature required for this is 375 – 550 Fahrenheit or 192 – 288 Centigrade depending on the amount of time that the heat is applied . This libun can be used to kosher utensils that normally would require immersion in boiling water (hagala), but cracks or crevices prevent the water from removing all food particles. It helps to burn and draw out some the hard-to-reach particles and absorbed tastes. [4a]
Which libun to use for ovens?
According to some opinions, our ovens require libun chamur – heating the inside of the oven with a blowtorch until it begins to glow. Obviously, this is not a very practical solution for home ovens. 
Other opinions hold that libun kal is sufficient  because the baked foods generally do not come into direct contact with the oven walls. Libun kal is achieved by turning on the oven to its highest setting for about 1 or 2 hours. 
Many poskim (halachic authorities) are uncomfortable with the option of libun kal for kashering the ovens and recommend that in addition to libun kal, one should line the insides of the oven with heavy foil Others advise not to kasher an oven at all due to the difficulty in properly cleaning it and applying libun chamur. 
Self-cleaning (Pyrolitic) Ovens
Pyrolitic ovens are self-cleaning ovens that heat up to about 500 degrees Centigrade (900 Fahrenheit). At this temperature, all food residues are reduced to an ash. Halachically, this is considered libun chamur by most of the more stringent opinions.
In theory, the self-cleaning cycle in a pyrolitic oven will both clean and kasher the oven, so there is no need to clean it first. Also, there is no need to wait 24 hours before koshering. However, there are some practical issues to consider.
The Institute for Science and Halacha in Jerusalem measured the temperatures during the self-cleaning cycle of a pyrolitic oven (Sauter) and found that while the inside of the oven reached the desired temperature (about 500 C), the metal areas surrounding oven door only reached 220-250 C. This is due to a rubber gasket that keeps the heat from spreading outside the oven. Therefore, Rav Levi Yitzhak Halperin, who heads the Institute, recommends removing the gasket during the self-cleaning process to allow the heat to spread to the corners of the door. However, this is not mandatory, because 220 C is considered libun kal, and that is actually sufficient for the corners of the door. In order for the libun kal to be effective, the outer rim of the oven and the all the areas surrounding the door including the gasket must be well-cleaned and stand unused for 24 hours prior to the self-clean cycle . (There is no need to clean the inside of the oven beforehand).
Please note that the above pertains only to self-cleaning Pyrolitic ovens — not continuous-cleaning ovens.
- THE STAR-K PESACH KITCHEN, Rabbi Moshe Heinemann, Star-K Rabbinic Administrator
- The Kashering Primer – PASSOVER, OU
- The Kosher Kitchen, A practical guide by Rabbi Binyomin Forst, ArtScroll – Mesorah Publications. Chapt. 15, “Kashering For Pesach”, in PDF format is available for download.
- Heavy and Light Libun: Does Temperature Affect Absorption? ,Rav Eliezer Melamed
- Koshering Ovens for Passover, Rav Eliezer Melamed.
 Stains that remain after use of a caustic cleaner can be assumed completely inedible and therefore no longer chametz.
 The Kosher Kitchen by Rabbi Binyomin Forst,Chapt. 15, “Kashering for Pesach”, p10. (see ‘Related links’ )
 This is an added precaution. By waiting 24 hours, any food we may have missed in the cleaning will become “spoiled” and not considered true chametz.
 אורח חיים, תנ”א, ד
[4a] Ibid. רמ”א, also see: Heavy and Light Libun: Does Temperature Affect Absorption? ,Rav Eliezer Melamed
 “Halachos of Pesach” By Rabbi Shimon D. Eider, page 180.
 Star-K: 40 minutes, Rav Tzvi Rimon: one hour, OU: an hour and a half (See related links)
 Kashering for Pesach, p. 9
 “Koshering an oven for Pesach is very problematic… On should refrain from koshering an oven for Pesach.”, Hakashrut, Rabbi Yitzchak Yaacov Fuchs, 2002, pp. 169-170
 Although Rav Halperin does not consider libun kal sufficient for the entire oven, he does think it is sufficient for the corners of the door because there are several leniencies which can be applied to those areas. For example, the edges of the door do not become hot with regular oven use due to the gasket around the door, so they would not absorb food directly. Source: “שימוש בתנור פירוליטי לבשר ולחלב והכשרתו לפסח” מאת הגאון רבי יצחק הלפרין, עטרת שלמה – קובץ שנתי מרכזי, תשס”ג, מכון מדעי טכנולוגי להלכה
 Kashering for Pesach, p.10
 Sidur Pesach Kehilchato, Rav Shlomo Zalman Grossman, p. 65. See also, Halachos of Pesach, p. 181
 “… There is no need for placing a piece of silver foil on the inside of the oven door. The food particles that might have gotten stuck there have no affect on any food cooked later on.” – Piskei Halacha of Harav Yisroel Belsky