What was found
HORIZONTAL STABILIZER TRIM (NORMAL AND ALTERNATE) INOP DURING DESCENT INTO ONT TRIM STOPPED AT 4 ANU STAB TRIM BEGAN OPERATING AT 4000 FT STAB TRIM OPERATED NORMALLY AT 4000FT. AUTOPILOT EICAS MESSAGE WAS DISPLAYED PRIOR TO STAB TRIM. REMOVED AND REPLACED STAB TRIM ACTUATOR AND JACKSCREW PER FIM 27-41-13-00. OPS CHECKS LEAK CHECKS GOOD P/N ON/OFF: 251T4310-1 S/N ON: KE0139 S/N OFF: AE0332