What was found
(CAN) DURING CRUISE ENGINE FIRE MASTER WARNING LIGHT, FIRE BELLS AND MASTER WARNING CHECK FIRE DETECTION LIGHTS CAME ON WITH NO OTHER INDICATIONS. FIRE BELLS AND LIGHT COULD NOT BE CANCELLED OR SILENCED FOR APPROXIMATELY 90 SEC. AFTER THAT FAULT DID NOT RETURN. ON INVESTIGATION BY ENGINEERING, ENGINE FIRE DETECT/CONTROL UNIT, SN 592 WAS REPLACED AND TESTED IAW TASK 26-11-00-710-801, NO DEFECTS EVIDENT. THE FAULT HISTORY OF PN 6504 SHOWS THAT ON 20TH MARCH 14, THE NR 2 ENGINE FIRE DETECTION AND CONTROL UNIT WAS REPLACED FOR FAULT FINDING, SN 592 INSTALLED. HISTORY FOR THE UNIT INDICATES THAT IT HAS BEEN PREVIOUSLY REMOVED FROM ACFT OTHER ACFT FOR SIMILAR DEFECTS.