A Bullet for El Diablo - S6-E10
Plot hole: During his investigation, Danno learns that the kidnapped Maria never wears make-up. Yet in every single shot we see of her, Maria is far from cosmetics-free. She's wearing very heavy eye make-up. (00:34:40)
Answer: He was fired. He never really appreciated his character. He felt Kono was portrayed as a big dumb Hawaiian and that the stereotype was racist. He also felt underutilized. He was fired after a heated argument with the show's publicist regarding his character. It seems there's not much details given regarding the incident or the firing, so it's seems possible he upset the show's producers as well.
Bishop73
No he said something derogatory about one of the Jewish producers that's why he was fired.