As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER 15 MINS OF CRUISE FLT AT FL400 (MACH TRIM OFF) AND (PRI STAB TRIM FAIL) MESSAGE KEPT DISPLAYING AND RESETTING. REMOVED DEFECTIVE ECM P/N 4214-0021-11N S/N 3125, INSTALLED UNIT MODIFIED BY MFG SERVICE, IAW MM CHAP 72-40-00. OPERATIONAL CHECKS GOOD.