What was found
AFTER APU START AND SEEMED STABLE PRIOR TO BLEED TRANSFER RECEIVED APU FIRE EICAS MESSAGE. JUST A FEW SECONDS LATER LIGHT WENT OUT. DID NOT USE FIRE SUPPRESSION SYSTEM. MX C/A: COMPLIED WITH BOMBARDIER RECOMMENDATIONS. GAINED ACCESS TO APU AND CHECK FOR ANY EVIDENCE OF A ACTUAL FIRE IN THE APU COMPARTMENT, NONE NOTED. COMPLIED WITH CONTINUITY TEST & INSULATION RESISTANCE TEST OF THE SENSING ELEMENTS IAW KIDDE AEROSPACE CMM 26-11-88, NO DEFECTS NOTED. AS PER ADVISORY WIRE REF AW300-26-0327, REMOVED FIRE EXTINGUISHER CONTROL UNIT AND INSTALLED A NEW CONTROL UNIT IAW CH 300 AMM 26-11-01-000-801/400-801. PERFORMED OPERATIONAL TEST OF FIRE DETECTION AND EXTINGUISHING (FIREX) SYSTEM IAW CH 300 AMM 36-11-00-710-801. OPS CHECK SATISFACTORY.