When my A321 powers up, the lower ECAM DU shows only a diagonal white line. What does it mean and what would be the fix?
Display Management Computer issue:
If a DMC fails, all information is removed from the displays it controls and replaced by a white diagonal line to indicate a working display with a failed source. ECAM will detect the failure and provide a sequence of alerts.
Due to the priority rules, the E/WD display will move to the lower DU if it was DMC1 that failed. The EIS DMC switch is used to replace the failed DMC with DMC3.
Steps to clear warning:
First select DMC #3 to isolate fault to DU or DMC. If DMC is at fault reset above associated circuit breaker for 5 seconds. If fault is with DU reset these (CAPT PFD/ND = G3/E1) (FO's PFD/ND = R3/R4) (Upper ECAM = E5) (Lower ECAM = R11).
More in the maintenance manual at AMM31-60-00-710-001. (EIS start).