What was found
WHEN CLIMBING, WITH THE AUTOPILOT ENGAGED, BETWEEN FL210 AND FL220, THE AMBER STAB TRIM MONITOR MESSAGE POSTED. WE RAN THE QRH AND MANUALLY USED THE PRIMARY STAB TRIM FOR AIRSPEED CHANGES. FODM WAS CONTACTED AND A DIVERSION WAS INITIATED. CORRECTIVE ACTION: REPLACED HORIZONTAL STABILIZER TRIM ACTUATOR CONTROLLER DUE TO AMBER STAB TRIM MONITOR CAS MESSAGE. REPLACED WITH OVERHAULED CUSTOMER SUPPLIED CONTROLLER. OPS CHECK GOOD. NO FURTHER DEFECTS IAW AMM 27-40-20.