I must have gotten this from visiting Peake's web site or something, because "DD" wasn't originally included in my Peake manual under Table 11 on page 20 of my Peake Manual, but I penciled in the following:
"DD = After cat O2 sensor response time too slow, cyl #4-6"
I hope that helps. There was probably a correction/addendum sheet that came with my Peake tool (R5/FCX-II), and I most likely penciled it into my manual rather than try to keep track of the separate sheet of paper.
As I recall, this table wasn't even applicable to my 540, but I'm pretty anal about data like that. I'm glad it was of use after all!
PS - Sorry this took so long to answer!
PPS - As to what to do next, given this code, I don't know what to tell you, except replace your O2 sensor if it is time or past time to do so anyway.