Kerias Shema consists of only 245 words. In order to say 248 words in , the chazzan repeats the words "ה' אלוקיכם אמת". If one davens without a minyan one should say "אל מלך נאמן" in order to reach 248 words. משנה ברורה, או"ח ס"א, ס"ק ו.
Hearing the words from the chazzan works with the concept of שומע כעונה. What is the halacha if a yachid is not up to where the chazzan is and as such, hears the chazzan saying ה' אלוקיכם אמת while the yachid is still in a different section of davening, for example פסוקי דזמרא?
Does the yachid have to say אל מלך נאמן upon reaching shema or can he rely on the chazzan, even though the yachid heard ה' אלוקיכם אמת while he was still in an unrelated section of davening?
Also, does it make a difference if the yachid heard the chazzan saying ה' אלוקיכם אמת while the yachid was saying Kerias shema vs. an unrelated section of davening?