TC reported fire external at front DT car. SM (YCK) checked and confirmed no sign of fire and no oth er abnormalities at the exterior of the car. Rear train T429(3620) performed line clear and feedback no abnormalities. ~~~~~ TC reported fire external at front DT ca     #3506 Fault code 721 (smoke detector) PCB wire terminal sign of water and smoke detector base rusty.Replace new smoke detector and function test ok. Done by Zulq/Saini/Haslan. 01/03/22