As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT CRUISE FL 340 #1 MCDU FAILED WITH BLANK SCREEN AFTER APPROX 5 SECOND BIT PASS APPEARED. THEN MCDU RESUMED NORMAL OPS. GROUNDED - YES. REMOVED & REPLACED #1 MCDU REF B737-900 AMM 34-61-01 & OPS CKS GOOD SW VERIFIED PER B737-900 IPC 34-61-01-03 GROUND VERIFICATION CK PASSED.