TIMS showed Smoke External Alarm at car1517. Staff(KTB) checked and confirmed no signs of smoke / f ire / burning smell on exterior or interior of train. Rear T 220 line clear and confirmed no abnormalities. TIMS showed S ~~~~~ 1517: To changeout External Smoke Detector  Found #1517 smoke external sensor very dusty. Changed out smoke detector and function tested ok. Notification: 2571940 Done by: Rizman/Koh/Jack 45339 1517 TIMS showed Smoke External Alarm. Staff(KTB) checked and confirmed no signs of smoke/fire/burning smell on exterior or interior of train. Rear T220 line clear and confirmed no abnormalities.  1517 FC 721 (smoke external). Checked #1517 external smoke detector found No plaseal, installed plaseal. Found smoke detector terminal side no gasket. OPS 20 raised for smoke detector c/o.     Sarma/Sugu/Alan/Shafie 45328