"Consciousness" is for Heidegger a too metaphysically loaded term , a term associated with a couple of metaphysical positions Heidegger wants to get rid of :
(1) The self is a " subject" ( " subjectum") , a substance ( a " thinking substance" according to Descartes) , an independent being ( " in se et per se" traditionnally ) --> Heidegger claims the self is " defined" by its relation to the world , its "openness" ( cf " being in the world" )
(2) The self understands being as " object" ( what is " in front ", what is " re-presented" ); being is reduced to "being known" in an oppositional structure. This " model" finds its completion in Hegel where the self has to oppose himself as object in order to know himself, in order to arrive at "true" consciousness --> Heidegger wants to show that we do not discover beings as " objects" known but as " tools" that we use, and with which we have an initmate relation ( without the " distance" caracterizing the notion of " objectivity").
(3) The self is primarily a "knower" ( since " conscientia" implies " scientia", knowledge) , cognition is the essence of self --> Heidegger wants to stress the practical aspect of daily existence
(4) The self is separated from others --> Heidegger claims that in daily existence, the self finds himself amongst others, defines himself as " one amongst the others"
(5) The self ( transcendental self) is this center that constitutes being as being, that " posits" being as being --> Heidegger wants to avoid the idealist implications of " consciousness" he finds in his Mentor E. Husserl ( cf Husserl's Ideen)