What was found
(CAN) ON APPROACH INTO DESTINATION THE AIRPORT, DURING IMC CONDITIONS, THE CREW HAD A POSITION UNCERTAIN STATUS MESSAGE ON BOTH FMS UNITS, THEY CONDUCTED A MISSED APPROACH AND RETURN TO THE DEPARTURE AIRPORT. THE FLIGHT LANDED SAFELY WITHOUT ANY FURTHER INCIDENT. ON ARRIVAL, MTC FAULTED THE `POSITION UNCERTAIN MESSAGE` TO THE #1 FMS. THE #1 CDU WAS REPLACED, FMS UPDATE CARRIED OUT AND TESTED WITH NO FURTHER FAULT NOTED. CDU PN:2017-41-211 SN:662 TSR:2816 CSR:2900 (TC# 20190408018)