AN A319 WAS DISPATCHED WITH #1 FIRE DETECTION UNIT SWAPPED WITH THE AUXILIARY POWER UNIT FIRE DETECTION UNIT DUE TO PARTS AVAILABILITY. ACFT DIVERTED DUE TO FALSE AUXILIARY POWER UNIT FIRE INDICATION.

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I WAS SENT TO GATE FOR A FIRE INDICATION ON #1 ENG. THE HANDLE HAD A RED LIGHT ON; SO I FIRST DID A FLT RPT AND A BITE CHK ON THE #1 FIRE DETECTION UNIT. THE RPT SHOWED NOTHING AND THE #1 FDU RPTED A #1 ENG LOOP B; NO OTHER INDICATIONS WERE PRESENT. WHEN PULLING THE #1 ENG LOOP B CIRCUIT BREAKER; THE LIGHT WENT OUT. I THEN CALLED MAINT CTL; AND HE TOLD ME TO MEL THE SYSTEM (B LOOP) AND TURN OFF THE B LOOP. I TOLD HIM YOU CAN'T TURN OFF THE B LOOP ON AN AIRBUS; ONLY BOEING. THEN HE TOLD ME TO PULL THE CB; COLLAR IT; AND MAKE A NOTE OF IT IN THE LOGBOOK. I TOLD HIM THAT WASN'T A SPECIAL PROC IN THE MEL BOOK. I THEN CALLED MY LEAD TO COME OUT. WE HAD NO FDU IN ZZZ; SO WE SWITCHED #1 WITH THE APU POS; THE PROB FOLLOWED AND THEN MEL'ED THE APU A AND B LOOPS; PULLED THE TWO CB'S AND SENT THE ACFT IN THAT CONDITION. THE ACFT WAS FLYING FROM ZZZ TO ZZZ1 AND DIVERTED INTO ZZZ2. WHEN RECALLING ECAM IN FLT; IT SHOWED A MESSAGE 'APU FIRE.' THE FIRE LOOPS CAN BE MEL'ED BUT NOT THE ENTIRE FDU; AND SINCE THE FALSE INDICATION ORIGINATED FROM THE FDU; IT SHOULD HAVE BEEN CHANGED BEFORE FLT; POSSIBLY GIVING THE PLT A FALSE INDICATION INFLT; MAKING HIM THINK HE NEEDED TO LAND ASAP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THERE WAS NO INDICATION ON THE FIRE DETECTOR UNIT OF AN INTERNAL FAULT. THE UNIT WOULD HAVE BEEN CHANGED IF ANOTHER WAS IN STOCK BUT HAVING NONE AVAILABLE; THE DECISION WAS MADE TO RENDER THE AUXILIARY POWER UNIT INOP AND DEFER PER THE MEL AND USE THE REMOVED FIRE DETECTION UNIT IN #1 ENG. FIXED #1 ENG FIRE DETECTION SYSTEM; BUT CAUSED A FALSE FIRE WARNING IN THE AUXILIARY POWER UNIT AND A DIVERSION.