Super strange. Can't find it in the service manual... But I think something-something-48-ish came up at a customer, and a flat cable to the printheads did the job. Can't say for sure though..
The problem with a flat cable replacement is that I don't think you can just replace the cable. On my 6400, the carriage unit was replaced along with the main board, but I do remember the carriage unit having the cable go right in with no ability to swap it out... it seemed to be a part of the unit.
From my experiences as well, when a new printhead doesn't solve the problem, it can just as easily be the cable, carriage unit, or main board. All the techs do is swap out the parts and see what works. The unfortunate thing is that if the main board is bad, but you just change the carriage unit and put in new heads, the bad main board might fry everything. When you change the main board, you have to change everything else down the line again.
I had the carriage unit replaced, didn't fix the problem. Had the main board replaced, didn't fix the problem. Now, the suspect was that even though the board was new, it fried the just replaced carriage unit, which would have fried the new board in return. On the third try, the tech put in a new main board, a new carriage unit, and 2 new heads. Problem fixed. Thank god warranty covered it.