TIMS showed Smoke External Alarm at carfront DT. Staff(AMK) checked and confirmed no signs of smoke/ fire / burning smell on exterior or interior of train. Re ar T407 line clear and confirmed no abnormalities. ~~~~~ TIMS showed Smoke External Alarm at car front DT (3542) . Staff(AMK) checked and confirmed no signs of smoke / fire / burning smell on exterior or interior of train. Rear T407 line clear and confirmed no abnormalities. CE: 3542 Smoke External Alarm Reset.Fault Cleared.  FC 721 Smoke external at car 3542. Checked external FSD at car 3542 and found PCB rusty and sensor very dusty. Changed out the FSD unit and function tested ok. External FSD: S/N:   Notification: 2513881 DOne by Jack, Sanip, Koh, Rizman 45149