TIMS showed Smoke External Alarm at car3556. Staff(AMK) checked and confirmed no signs of smoke / f ire / burning smell on exterior or interior of train. Rear T 431 line clear and confirmed no abnormalities. ~~~~~ TIMS showed Smoke External Alarm at car #3556 fault code 721x1 time.Further check found smoke detector PCB base rusty.Replace smoke detector unit done.Check #2 end all car plaseal-ok.Functional check smoke detector ok. #3556 smoke detector unit:- Not: 2483444 raise to CSC Done by Saini/Zulq.  27/04/23