TC reported Rear DT car B3 door not proven closed. TC open and closed train doors a few times to no avail. Staff (SBW) assisted to push to closed-ok ~~~~~ L3 3644 B3 door FOD check - OK. Door open/close operation check - OK. Push-pull gauge check & open/close timing check (within criteria) - OK.  Selvan/Anand/Alan. 21/02/2020. L3 3644 B3 door operator removed & found 2 screws broken. Drilled & re-tap the threaded hole - Done.  Selvan/Anand. 24/02/2020. L3 3644 B3 door operator swopped with 1644 B3 door. Function test - OK. Door gauge test, push/pull gauge test & door open/close timing check - OK.  Chan GF/Selvan/Anand. 25/02/2020.