What was found
THE PILOT REPORTED THE EX600 MFD DISPLAY REMAINED DIM AND NOT LEGIBLE FOR APPROX 10 TO 15 MINUTES DURING THE ACFT GROUND RUN-UP CHECKS PRIOR TO TAXI. WHEN THE DISPLAY ACHIEVED 'NORMAL' BRIGHTNESS AND WAS READABLE IN DAYLIGHT CONDITIONS, ALL THE ASSOCIATED FUNCTIONS PERFORMED IN A NORMAL MANNER. A WARRANTY EXCHANGE MFD IS BEING PROCURED. THIS IS THE SECOND EX600 MFD WITH A DISPLAY PROBLEM WE HAVE ENCOUNTERED IN THE PAST 6 MONTHS ON THIS ACFT. THE AVIONICS SHOP SUPPLYING THE WARRANTY REPLACEMENT MFD STATED THEY KNOW OF 4 OTHER UNITS THAT HAVE EXHIBITED SIMILAR PROBLEMS.