1}Engine Bd. and DC cont. have the NVRAM. They have to get in sync. If one is corrupted, it can ruin both, becaus they cross-read their info.
2} If one or the other is replaced, the NVRAM may still get messed up. Knowing for sure which one is really the bad one is hard, and Xerox suggests replace both.
3} Sometimes several re-starts are necessary for them to get in sync.
"Impatience will reward you with dissatisfaction" RMS Cosmics'97