What was found
(CAN) PILOT REPORT, AFTER ACFT BOARDED AND AN ATTEMPTED APU START MADE A CHECK FIRE DETECT WARNING WAS OBSERVED ALONG WITH A GEN OVERHEAT ADVISORY LIGHT. FUMES AND SMOKE FROM THE APU AREA WERE CONFIRMED. NO APU FIRE ADVISORY WAS PRESENT. ACFT WAS DE-POWERED AND DISEMBARKED. SMOKE AND FUMES DISSIPATED AND NO FIRE WAS OBSERVED. ACFT DECLARED U/S AND HANDED OVER TO ENGINEERING. PAX DEBRIEFED. MAINTENANCE REPORT, APU STARTER/GENERATOR REPLACED AND OP CHECKS C/O SERVICEABLE IAW AMM 49-10-00, CONTACTOR 2449-K62. REPLACED IAW WDM 24-49-01. GCU REPLACED AS PRECAUTION IAW AMM 24-31-16.