As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SINGLE YELLOW VERTICAL LINE DISPLAYED IN THE CAPT'S FMC SCREEN- FMC SEEMS TO OPERATE NORMALLY - NO MESSAGES DISPLAY ON SCRATCH PAD. LATER IN FLIGHT VERTICAL YELLOW LINE DISAPPEARED. GROUNDED - YES. REPLACED CAPT FMC CDU WITH SERVICEABLE ONE. VERIFIED CDU SOFTWARE. ACCOMP MCDU INSTLN TEST, TEST PASS REF B737-800 AMM 34-61-01-400-802L.