As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ORD - EN ROUTE ORD-DCA LEVELING OFF AT 5000 FEET WITH AUTOPILOT ON, AUTOPILOT TRIM LIGHT CAME ON WITH NOSE UP PITCH. DISCONNECTED AUTOPILOT AND RETURNED TO ORD AND LANDED WITHOUT INCIDENT. REMOVED AND REPLACED ALT STABILIZER TRIM MOTOR. SYSTEM GROUND CHECKED NORMAL OPERATION.