The Torah must be read from a kosher scroll. This means that it must be a Torah scroll written for the sake of Heaven, with ink on parchment, just as the first Torah scroll was written by Moshe Rabbeinu, according to the direct word of Hashem. Even a congregation which sets aside time for Torah study must assemble at least every three days to read from the original Torah scroll, just like the Torah which Hashem gave to Moshe on Mount Sinai. If there is no minyan present, the mitzvah of reading the Torah cannot be fulfilled, since it is a matter of sanctity requiring ten Jews.
If the Torah scroll is missing even one letter, the entire scroll is invalid, and it cannot be used to perform the mitzvah of Torah reading. This halachah emphasizes the extraordinary quality of the Torah. Its entirety is one complete Divine idea which is expressed by all of its narratives, mitzvot, teachings, and letters together. If even one letter is lacking, a flaw exists in the absolute completeness of the Divine Torah.
According to a number of Rishonim, it is permissible, b’dieved, to read from an invalid Torah scroll (Rambam’s responsa to the sages of Narvona; Mordechai). Still, in practice, the opinion of the majority of poskim is that the mitzvah of Torah reading can only be fulfilled with a kosher Torah scroll, and that is the halachah (Rashba, Rosh, and Rambam in his halachot; Shulchan Aruch 143:3).
If, in the middle of the reading, a faulty letter is discovered which renders the Torah scroll invalid, a different Torah scroll is taken out and the reader proceeds from where he stopped in the first. We do not continue reading from the first Torah since, according to the majority of poskim, it is forbidden to read from an invalid scroll. On the other hand, we do not require the congregation to repeat the beginning of the Torah portion, since b’dieved we rely on the poskim who maintain that the obligation of Torah reading can even be fulfilled with an invalid Torah scroll.[1]
If a defect is found in one of the letters, and there is doubt as to whether or not it invalidates the Torah scroll, the reading is still continued from that Torah. There are two reasons to act leniently in such a case. First, it is possible that the Torah scroll is actually kosher. Second, even if the Torah does possess something which renders it invalid, we have already learned that there are opinions which maintain that b’dieved it is permitted to read from an invalid Torah scroll. Still, the Torah must be repaired promptly after the reading.
[1]. If the mistake is found in the middle of the reading, according to the Shulchan Aruch, the invalid Torah scroll must be switched for a kosher one, and at least three verses must be read from it so that the one who was called up may recite the blessing after the reading on the kosher scroll. If there is no kosher Torah scroll available, the berachah after the Torah reading is not recited on the invalid scroll. According to the Rama, if the reader reached a point at which it is possible to end the reading (which is not less than three verses from the beginning or end of a section), the blessing after the reading is recited on what he read from the invalid scroll, so as not to switch scrolls in the middle of an aliyah. However, if the mistake is found in a place in which it is not possible to stop reading, since it is not permissible to continue reading from an invalid Torah scroll, they must switch scrolls and continue reading from a kosher one, and the person called up to the Torah recites the blessing after the reading on the kosher Torah scroll (Shulchan Aruch Orach Chaim 146:4; Mishnah Berurah 13).
