As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
STAB TRIM CAUTION AND MACH TRIM CAUTION MESSAGE DISPLAYED. FOLLOWED QRH TO TRY TO ENGAGE. WHEN F/O USED TRIM A STAB TRIM AND MACH TRIM CAUTION MESSAGE AGAIN DISPLAYED. REPLACED STAB TRIM ACTUATOR AND MOTOR CONTROL UNIT. OPS CHECKS GOOD ON BOTH.