TC reported intermittent External SmokePre-Alarm 2 in Rear M2 car. Staff JUR checked and confirmed no abnormalities found. ~~~~~ 2509 external smoke detection reset - OK. Swopped fire control panel to 1509 - Done. Monitor for 3 hrs & no fault occurred.     Chan GF/Alcus. 05/10/2020.  2509 found smoke detector faulty. Replaced external smoke detector with available spare. Security/Function test - OK.  Kana. 05/10/2020.