What was found
20 MINUTES INTO FLIGHT, CAPTAIN`S NAVAGATION DISPLAY BLANKED OUT AFTER ABOUT 5 MINUTES, THE LOWER CENTER DISPLAY BLANKED ALSO A FEW MINUTES LATER BOTH DISPLAYS RETURNED SIMULTANEOUSLY FOR A COUPLE OF MINUTES THEN BOTH BLANKED AGAIN NEVER TO RETURN. AIR TURNBACK. NO EICAS ASSOCIATED WITH MALFUNCTION. INTERROGATED THRU MAT FOUND ACTIVE MX MESSAGE 31-80221,31-80222,31-80223. R & R CAPTAIN`S NAVAGATION DISPLAY UNIT IAW AMM31-61-01-4 AND TESTED SATISFACTORY. ALSO FOUND NO ACTIVE MESSAGES. ACFT OK FOR SERVICE. TESTED SATISFACTORY AND MAT SHOW NOT ACTIVE FOR THE MX MESSAGES, FAULT HAS BEEN CORRECTED. OK FOR SERVICE.