What was found
(CAN) UPON ATTEMPT TO CENTER THE RUDDER, THE FLIGHT CREW REPORTED THAT THE RUDDER TRIM SWITCH BECAME UNSERVICEABLE SHORTLY AFTER DEPARTURE. UPON REFERRAL WITH MOC IT WAS DETERMINED THAT THE BEST COURSE OF ACTION WOULD BE TO RETURN AND HAVE IT SERVICED PRIOR TO NEXT DISPATCH. ON ARRIVAL, PASSENGERS REMAINED ONBOARD WHILE MAINTENANCE REPAIRED THE ITEM. AN ENGINE RUN WAS REQUIRED AT THE GATE. ONCE COMPLETED & SYSTEMS CHECKED, ACFT CONTINUED BACK TO SERVICE TO OPERATE THE ORIGINAL SCHEDULE. MAINTENANCE ACTION: RUDDER TRIM CONTROL UNIT WAS REPLACED. FUNCTION CHECKS COMPLETED. ACFT WAS DEEMED SERVICEABLE AND RETURNED TO SERVICE.