What was found
INBOUND ATW CREW PERFORMED START ON APU & RECEIVED APU FIRE WARNING FOLLOWED BY APU AUTO SHUT DOWN. AFTER SHUT DOWN OF APU, WARNING EXTINGUISHED & NO FIRE SUPPRESSION WAS USED. EMERGENCY WAS DECLARED INBOUND ATW, NO EVIDENCE OF FIRE FOUND. INSPECTED APU, FOUND NO EVIDENCE OF FIRE & DEFERRED APU FIRE DETECTION LOOPS IAW MEL 26-12-01. TROUBLE SHOT APU FIRE DETECTION SYS IAW CRJ200AMM 26-11-01. FOUND FIRE, OVERHEAT CONTROL UNIT INTERMIT. R & R FOCU IAW CRJ200 AMM 26-11-01, OPERATIONS GOOD IAW CRJ200 AMM 26-12-00, THIS CLEARS DM 1789, PLACARD REMOVED, OK FOR SERVICE, CLEARED FORM MM02.