When one says shema it must contain 248 words. At times a person has to say shema without a minyan in his prayers (or before the minyan or later (maariv) for zamanim issues or shema before sleeping ). Whats the best way l’chatchilla to get the extra 3 words? Do we say “Kel melech ne’eman” at the start or do we repeat “Ani adon… elo-heichem” at the end another time? Isn’t “Kel melech ne’eman” a hefsek after birkat shema?
Although the Rema writes (61:3) that one who is davening without a minyan should add the words “e-l melech ne’eman” before Shema, the common custom is not to say these words, because of the concern that this constitutes a hefsek (interruption) between the blessings of the shema and the recitation of the shema. Rather, the words at the end “Hashem Elokeicheim Emes” should be recited twice (according to the Vilna Gaon the word Emes is not recited twice), completing the 248 words.
Sources: See Tur 61, quoting at length from Rama, and Beis Yosef; see Mateh Moshe (90), and Magen Avraham (90:3). It is possible that the reason we do not follow the ruling of the Rema concerning this issue is that the words E-l Melech Ne’eman are in place of answering amen, and since many are concerned not to answer amen before Shema (the common custom, based on Peri Megadim, is to complete the blessing before Shema together with the shaliach tzibbur), we also avoid saying E-l Melech Ne’eman.