in chapter 3 only
Nick's value as a narrator and the voice of traditional morality in the novel is founded upon his personal integrity. When Nick goes East, he is non-judgmental in his view of others' conduct; when he returns, he rejects the selfishness and amorality he has witnessed. In Chapter I, Nick says he doesn't care what human behavior is founded upon, so long as it is founded upon something. He comes home wanting the world to stand at "moral attention." These are not the feelings and reactions of a dishonest man.
Nick's basic honesty is seen in his relationships with women. After he goes away, he makes sure that the girl he left back home understands that their relationship is over. He does not want her to misunderstand his absence or their relationship. He is honest with her, although he could have simply left. When he decides to go back home, he goes to see Jordan, although it is not a visit he looks forward to. Nick sees it as "an awkward, unpleasant thing," but he wanted to act responsibly, "to leave things in order." Again, he chooses not to be dishonest with her or to simply walk away.
Nick's only acts of dishonesty, if it they can be considered as such, would be his arranging the meeting between Gatsby and Daisy and his going with Tom to the party in New York where he meets Myrtle, Tom's mistress. He does not tell Daisy about his trip to Tom's New York apartment, and he does not tell Tom about Daisy's resuming her relationship with Gatsby. However, Nick is not Tom's and Daisy's keeper; it is not his role to police their conduct. And since Nick is a functional character in the novel, had he not played a role in these two incidents, the story would not have progressed. They are, therefore, less a reflection on Nick's honesty than they are necessities of the novel's narrative structure.
Nick's opinion of his honesty is largely based upon the people that he is surrounded with. He is telling this story of Gatsby after it has taken place, therefore the people that he has met in East Egg, West Egg, etc. have a great influence on how he sees people and their honesty. Almost every character that he has met up with in the novel is dishonest whether it involves cheating at work, cheating on a spouse, lying to another person, gambling, etc. Therefore, when Nick thinks about these types of people, he feels that he is nothing like them and deems himself honest.
Nick's honesty is based upon the reader's interpretation of Nick as a narrator. All of the information that he is getting, comes from the people that he says are dishonest. He then passes this information to the reader. Therefore, Nick seems to be honest in giving us the information that he has been given but the reader must decide whether the information that Nick is getting is reliable. That is the only way to determine Nick's honesty.