Do journalists have an obligation to check facts, or should they merely repeat what they hear?

It has long amazed me to see that usually, when an interviewee blatantly lies or makes an incorrect statement, the journalist will let it stand. To me, this undermines the credibility of the journalist. yet it is not seen as the function of most journalists. It seems that the justification for the special position of the Fourth Estate is the fact that it seeks the truth to put in front of the public. Yet it has become more like entertainment. If journalists are not charged with seeking the truth, and challening untruths, why should the press occupy a special position?